I have never stayed in either hotel (I have a flat I London!), so can't comment from that point of view, but they are both pretty high end hotels. The Westminster Intercontinental is very new - I had to look it up, it's in an old building that I knew as government offices
The areas are very different. Westminster is primarily offices, but very close to some of the common sights - a few minutes walk to the Abbey, Buckingham Palace, 'Big Ben', the river, London Eye etc. Trafalgar and Leicester Squares are not too far away. It is literally across the road from St James's Park tube station, handy for buses and so on.
The Sheraton Tower is in Knightsbridge, an area famous for luxury shopping and hotels. It is near Hyde Park, the largest of the Royal Parks, but further away from the main tourist sites. But you are only a few stops on the tube from anywhere you are likely to want to be (as you are anywhere in Central London).
On balance, in London for a short time to sightsee, I would probably stay in Westminster (lots of cruise critics stay along the Victoria-Westminster-Waterloo axis, it seems) - you will probably feel more in the centre of things. But I'm probably biased, as I used to live just over the river in Vauxhall! And I'm sure the Sheraton would be a great place to stay too