Architecture: Above: a great place for a gallery: When it comes to picking a site for the Tate's next project, Bankside Power Station should be the choice, and not the more troublesome South Bank, says Jonathan Glancey

Here is Bankside Power Station described in Sir Nikolaus Pevsner's Buildings of England: '1957-60 by Mott, Hay & Anderson (engineers) and Sir Giles Gilbert Scott (architect). The last of the huge brick power stations which began with Battersea (see Battersea, Public Buildings), this was built to burn oil, not coke. Symmetrical, with central square tower-like chimney 325ft high. Large windows to E, W and N, a smaller horizontal band to the S. But the main impression is of the stunning scale of the bare walls of immaculate brickwork, excellently set off by the smooth green sward beside the new riverside walk.'

This was written in 1983. Eleven years on, Bankside Power Station sits magisterial and redundant on the south bank of the Thames. It faces St Paul's cathedral, across the river, and demolition if a new use is not found for it soon. Its one great hope is that it might yet become the new Museum of Modern Art proposed by Nicholas Serota, director of the Tate Gallery.

It would cost about pounds 80m to convert Bankside Power Station into an enthralling multi-media art gallery; Mr Serota is confident that he can raise half of this through private donations and the rest, he hopes, through the National Lottery and Millennium Fund. The lottery begins next year.

The Museum of Modern Art project, however, faces competition for these new funds: the redevelopment of the Royal Opera House in Covent Garden, for example, or the remodelling of the down-at-heel South Bank; equally they might be channelled into a new opera house in Cardiff for Welsh National Opera. The National Lottery promises to live up to its name: whether Nicholas Serota for the Tate or Nicholas Snowman, the director of the South Bank, scoop the tens of millions they need to advance their equally plausible plans depends on cupidity and the turn of fortune's wheel.

Peter Brooke, Secretary of State for the National Heritage, has made it clear that the first flush of state lottery funds will allow only one major cultural building to be financed in each of England, Northern Ireland, Scotland and Wales. So cash for Nick Serota means penury for Nicholas Snowman and vice versa.

Assuming that the Museum of Modern Art wins lottery funding, there remains the parlour-game question of where it should be sited. The Tate makes its choice public at the beginning of next month, but it is no secret it would prefer either Bankside or the South Bank.

Not everyone thinks these are the ideal choices: an exhibition at the Royal Institute of British Architects that opened in London yesterday ('New Heart for Vauxhall') locates the museum in Vauxhall, directly opposite the Tate Gallery. A reticent design by the architects Stanton Williams, however, has little of the sheer visual power necessary to stop the Tate in its tracks at this late stage. While it might make some sense to build the new museum directly opposite the Tate and, perhaps, to link the two by a pedestrian bridge, the site facing the gallery is occupied by a hotchpotch of dull commercial buildings and is unlikely to become free for years.

So, South Bank or Bankside? In my heart, I would choose the South Bank and commission a design from an architect briefed to create an uncompromised and truly thrilling building. In wilder moments, I might even ask Oscar Niemeyer, the veteran Brazilian architect, to conjure some soul-stirring sculpture of a building, or else turn to an up-and-coming British architect with a real empathy for modern art.

Tadao Ando's name has been reverentially mentioned, but where this most subtle of Japanese architects might have been ideal for the Tate at St Ives (or the next Tate venture in East Anglia), the exquisite qualities of his work would be lost in the constructional hurly- burly of the South Bank.

Sometimes, however, the head must rule the heart: the South Bank could well prove to be a headache for the Museum of Modern Art. This is not because of rivalry between the South Bank Board and the Tate trustees (Nicholas Snowman, while fighting for lottery funding, welcomes the idea of the museum on the South Bank), but because building here for technical and planning reasons will be difficult and slow.

Much money and time will inevitably go on fitting the building into the site (it will have to be dug deep into the South Bank sand). And, if the South Bank is not improved at the same time (not for a lack of trying on Nicholas Snowman's part), then the museum might end up as a bright jewel in a rusting crown.

No, the head says Bankside. Throw a beautiful new bridge designed by the most imaginative engineer teamed with one of the best lighting artists across the Thames from Christopher Wren's house of God to Giles Gilbert Scott's temple of power. Have an architect of real boldness and imagination sweep into the empty brick cavern and transform it into a thrilling place for the modern arts. Light it beautifully, make the entire building an artwork. The right architect will heighten, not spoil, the sheer power of Scott's building, currently exempt from listed building consent. Bankside is not a polite or a genteel building; it needs an architect who can rise to its challenge, not one who will play safe.

The choice of Bankside will also please the conservation lobby; the museum will save Scott's potentially sublime building from the sad fate of Battersea Power Station. While this should not be a prime consideration for the Tate, it does mean that the path to the opening of the new museum will be a far smoother one than it might otherwise be. Bankside is also there, hundreds of thousands of cubic feet of sublime and well constructed space.

Does this sound cowardly? I hope not. If we - or Nicholas Serota and the Tate - want to have a Museum of Modern Art built by the turn of the century, the choice of Bankside makes convincing sense. As the Tate empire expands, there will be a chance to employ some of the world's best architects on sites better suited to their talents. Yet, even if the choice of Bankside is in some ways a reluctant one, the right architect working to the right brief may yet prove that compromise can truly become art.

(Photograph omitted)