Was recently in London and stayed at the Park Plaza Westminster and also stayed at the Holiday Inn Mayfair. I would concur with drdrew that I really liked being near the river, jubliee gardens and the whole area. There are advantages such as pool and such at the Westminster. Tube stop is a small walk.

Of course if you want to be in the West End, the Mayfair is a great choice and seemingly a more luxurious choice.