I'd say the Park Lane and Trafalgar are both equally convenient to walk to most theatres in the West End. PArk Lane is also more convenient for upscale shopping (Bond Street).

The Trafalgar is cantrally located on Trafalgar Square, but is strangely isolated from shops etc - it's only a short walk but you sometime get the impression you're on a traffic island. I also feel the lobby bar is too noisy and tries too hard to be trendy.

But there again, I prefer Starwood...

For about the same price as you're being quoted by Hilton, you could stay at the Savoy - which I feel is the best hotel experience in London.