The short answer is that both are by Scottish artists and that paintings by the four so-called Scottish Colourists, such as Samuel Peploe, who painted Girl in White in 1909, are leading prices for Scottish paintings - highland cattle included - by the nose.
Those big-horned beasts may be dishevelled, mired and standing in the rain at the cheaper end of the market. But their unique selling point - so obvious, but missed by art buffs who still make jokes about granny's sitting room as they pass them by in the Portobello Road - is that they are ethnic.
And Scottish ethnic - don't ask me why - is more potent than English ethnic, at least in the art market. In London, Sotheby's and Christie's regularly pick out Scottish pictures consigned to their Modern Brit sales and bundle them off to their Scottish salerooms. They know that Scottish pictures in general have acquired an international following - among the rich MacWhatsits in America and the highland romantics on the Continent - whereas Modern Brits, apart from the big four (Spencer, Bacon, Auerbach, Freud), have not.
Even a watercolour of a sun-drenched Spanish bullfighting scene - not an image likely to appeal to an Aberdeen Angus - has been given a place in Sotheby's sale of Scottish pictures at Hopetoun House, West Lothian, on 20 April. It was painted by a Scot, Joseph Crawhall (1861-1913). Estimate: pounds 15,000-pounds 20,000. Scots wha' hae.
The Colourists are Scotland's big four: besides Peploe (1871-1935) there was John Fergusson (1874-1961), Leslie Hunter (1879-1937) and Francis Cadell (1883-1937). Dour they were not. They are Britain's post-Impressionists - bright, vibrant and painterly.
Small wonder, then, that collectors of Impressionist art - Americans, continentals and the Japanese (whenever they have any money) - have taken a shine to the Scottish Colourists. But try finding the Colourists' names, or even "Scottish Colourists", in art reference books published south of the border, and you will get some idea of how Sassenachs have sidelined Scottish art.
Impressionist prices, of course, lead the peaks and troughs of the art market. And now that Impressionist prices are creeping up again, following the recession, so are the prices of the Impressionistic Scottish Colourists and the prices of Scottish pictures in general. The ethnic dimension - expressed in the rise of Scottish nationalism - can only help boost the market.
Back in 1988, that Girl in White by Peploe fetched a sensational pounds 506,000 at Christie's sale of Scottish Colourists in Glasgow - five-times estimate and still the record price for a Scottish Colourist. It had been cunningly put on pre-sale view among French Impressionist paintings awaiting auction in London.
Christie's Alexander Meadowes had launched dedicated Colourist sales in Glasgow the previous year. His sumptuous hardback sale catalogues of Colourists were issued annually until after the crash of 1991. And what a crash it was. The Colourists went down the drain with the Impressionists. Prices halved. Christie's Colourist sales were axed.
But, like the Impressionists, they are making a comeback. Mr Meadowes resumed his Colourist sales in Glasgow last November, when 56 lots raised a total of pounds 772, 707. A Peploe, Pink Azalea and Oranges, made pounds 89,500. Not a patch on the hardback years, during which the 1988 sale raised pounds 1.8m. But the sale was a 100 per cent sell-out.
Which leaves those woolly cattle a little less mired. Christie's next general sale of pictures in Scotland is not until 28 May in Edinburgh (when the only known life-size portrait of John Brown, Queen Victoria's gillie, will be estimated pounds 100,000 - pounds 150,000) and Christie's Colourists not until 12 November in Glasgow. But at Sotheby's sale at Hopetoun House on 20 April there are some estimates for cattle-in-landscape paintings that can only be called bullish.
In the Highlands - big, 50in by 40in - by one of the leading painters of highland cattle, Louis Hurt (1856-1929), is estimated pounds 20,000-pounds 30,000. Three years ago, pounds 12,000-pounds 18,000 might have been expected. Two similar but smaller paintings of Hurt's are estimated pounds 3,000-pounds 5,000 and pounds 3,000- pounds 4,000. Only pounds 2,000-pounds 3,000 would have been hoped for three years ago.
At Christie's general picture sale in Edinburgh last November, a magnificent Breanski made pounds 32,200 - above the pounds 20,000-pounds 30,000 estimate. And a 20in by 30in Hurt - similar to the one being offered by Sotheby's - made pounds 19,550, well above its pounds 8,000-pounds 12,000 estimate.
On the coat-tails of Hurt there is William Watson (flourished 1866-1900, died 1921). His prices have begun to move up only in the past year. He is less atmospheric than Hurt, but his Highland Cattle Watering is estimated pounds 4,000-pounds 6,000. A year ago, Sotheby's would have been happy to get pounds 3,000- pounds 5,000 for it.
The painting to watch is the Peploe Still Life With Apples of about 1925, which is estimated pounds 40,000-pounds 60,000. That is what it might have fetched 10 years ago. Are prices for the Colourists nearing their peak? Not yet. The sign of that happening will be when Peploes in the pounds 250,000-plus price range start to venture into the saleroom. Until then, unless the economy drags the market down, as it did last time - which is unlikely - prices will continue to head north. So should you.
Sotheby's Scottish Pictures sale is at Hopetoun House, South Queensferry, West Lothian on Monday 20 April (6pm). Inquiries (0171-293 5000) and from 18 April (0131-319 1424/5). Christie's Scotland (0141-332 8134).