They have much in common. Both have headed their own companies for 20 years and seen them grow into a colossus. Both are seen as visionaries and both are violently rich. But there are differences too. Gates is still essentially a geek with little to tell us beyond computing and Windows. Ellison, by contrast, 52 years old and thrice-divorced, can kindly be said to be have a large head and a willingness to expose his ambitions. Among those - as he told Oprah Winfrey on television - is to find new love.
But there is more that separates the two men. Until recently, few of us had even heard of Ellison nor had much idea of what Oracle did - even though it had grown into the world's second largest software company, after Microsoft. Its sphere is software for database management. With giant corporate clients like American Airlines, British Telecom and Barclays Bank, Oracle is the dominant provider of software for systems handling mass reservations or interactive video tele-conferencing. Gates we all know. And Gates remains richer - $18bn (pounds 11bn) in estimated personal worth against Ellison's $6bn - and more powerful.
This is what gnaws at Ellison. His obsession with displacing Microsoft has become increasingly plain. Watch for Windows-knocking rhetoric when Ellison addresses the annual conference of European Oracle users in Vienna tomorrow. "Microsoft's job is to get Windows everywhere," he said at one such gathering last year. "Our job is to stop them." The battle enters his personal life too: not to be outdone by Bill Gates's much-trumpeted and gizmo-packed new home under construction outside Seattle, at a cost of $30m, Ellison has embarked on a new spread for himself on San Francisco Bay. It is modelled on a Japanese monastery and is budgeted at a yet more outrageous $40m.
What impels us to take Ellison seriously is his commitment to the NC, or Network Computer. Reaching the shops about now, these are the cheap and cheerful machines with which Ellison hopes to topple the personal computer and with it the Wintel standard of Microsoft Windows and Intel's chip technology. What is different about an NC is that it fuels itself from outside, either from the Internet or from an internal company network. An NC does not have its own memory capacity or even a hard drive - nor does it have a slot for CD-Roms. It uses the Internet to get information and to store it.
Since first speaking of his NC vision two years ago, Ellison has assembled a potent coalition of industry leaders, now part of the anti-Microsoft camp. His recruits include Apple, IBM and, and in particular, Sun Microsystems. And the NC represents a serious challenge simply by virtue of its cheapness. Each machine is priced at around $500 - less than a quarter of that of a self-respecting PC. Ellison likes to point out that when they buy a PC, most people are getting a machine too complex and expensive for the tasks it will perform. Corporate buyers, meanwhile, are sure to be interested in equipment for which the maintenance costs will be a fraction of those for full-blown PCs.
Then there is the occasional guest-star role played by Ellison in the endless soap opera of Apple. Ten days ago, he burst back on to the set, announcing that he was scouting for investors who may be interested in joining him in a bid for a controlling interest in the ailing Apple computer. He even invited shareholders and customers to offer their thoughts on the subject to him at a specially-created e-mail address. It is safe to say that if Ellison manages to yoke Oracle to Apple, everyone - Mr Gates included - will sit up and take notice.
All this is not too shoddy for a boy from Chicago's rough south side, who was abandoned at birth by his mother and raised in humble circumstances by a great-uncle and great-aunt, Louis and Lillian Ellison. Louis was a first-generation Russian immigrant who named himself in America after Ellis Island, the first stop for so many of the huddled masses. Larry, who has one adoptive sister 15 years his senior, is remembered as a sparky but somewhat unruly boy, who liked to "laundernaut" in the nearby laundromat - these days he prefers yachting and water-skiing to spinning around inside dryers - and who never focused on his academic career. It was only after dropping out of two universities in Chicago that Ellison took to the road in his 1964 Thunderbird and hit California.
The year was 1969 and two things were under way. Flower power was at its peak - Ellison likes to tell interviewers that, unlike his friend, Bill Clinton, he did inhale - and the promise of Silicon Valley was just becoming apparent. Ellison worked as a programmer in different companies until 1977 when he persuaded his late friend and business partner, Robert Miner, to set up a consulting company. With $1,600 between them, they founded Software Development Laboratories, later to be Oracle.
Ellison likes to say that his principal interest is in the technology of the computer business, not in management or sales. He nonetheless became a legend in Silicon Valley for the ruthlessness of his sales targets. Reps who delivered were richly rewarded, those who did not were discarded. Sales doubled every year through the 1980s and the database competition was left gasping. But it was a sort of mania, created by a distinctly dictatorial Ellison, that almost brought the company down.
By Ellison's admission, Oracle was at the brink of bankruptcy in 1990 when it was discovered that its sales team had been booking revenues before they were in, and even selling products before they were available. In the meantime, Oracle had gained a damaging reputation for sharp elbows if not outright crooked business practices. The ship was righted when Ellison acknowledged his shortcomings and brought in new blood at the helm, notably Raymond Lane, as USA president, and Jeffrey Henley as chief financial officer. By May 1996, Oracle's revenues had topped $4bn.
Today Ellison gives the impression of a warrior - in keeping with his interest in things Japanese, he would probably suggest a 16th century Samurai - preparing finally to storm the Microsoft citadel. Very soon, we will have a clearer idea of the true potency of his most important weapon, the NC. "We are the critical moment," says William Milton, an analyst at Brown Brothers Harriman in New York. "This battle is just getting under way. But I think the NC has to be a threat."
Much more urgent now is the Apple question. Infamous for abandoning the speeches that have been prepared for him and for blurting out what is on his mind, Ellison may guide us when he takes the microphone in Vienna tomorrow. What we know so far, thanks to an interview he gave last month to the San Jose Mercury newspaper, is that he is talking of a bid of about $1bn to take a 51 per cent stake - if he can find anyone else who is interested. Ellison has said that his first move at Apple would be to replace management and in particular the chairman of only 13 months, Gil Amelio. The person he would elevate in Amelio's place would almost certainly be Steve Jobs, co-founder of Apple, who recently rejoined it as a consultant. Jobs and Ellison happen also to be old friends.
Additional intrigue has been provided by last week's news of a five per cent investment in Apple by Saudi Arabia's Prince al-Waleed bin Talal. It is not assumed that the Prince, who has reputation for putting money in struggling businesses with potential for recovery, and Ellison are on opposing paths. "I know Ellison," the Prince told the New York Times. "I like and respect him."
Milton, who follows Apple in particular, notes the eccentricity of Ellison in announcing his interest to a newspaper and triggering a rise in the Apple share price. But he, among others on Wall Street, suggests we take the Apple manoeuvre seriously. "Investors would like to see Larry Ellison and Steve Jobs running that company," he concedes.
But are they not also a little of wary of Ellison and his overblown designs? Says Milton: "Silicon Valley is filled with oversized egos, and it just happens that this one also produces."