I've stayed at both Grosvenor Square and Marriott Kensington.

Grosvenor Square is immediately by both the US and Canadian embassies, and it is convenient to Hyde Park, Mayfair, Buckingham Palace, the British Museum, etc. Facility was typical of a European hotel, rooms tended to be smaller than the US, but comfortable. Service was good. The proximity to the embassy can be either a positive or a negative, depending on the state of political dissent.

Kensington is a bit further out. It is more convenient (but not immediately close to) to Sloan Square (shopping), Harrods, and the V&A. There was a bit of a wait on a couple of occasions for a taxi: the hotel is along the main road to LHR, and many of the taxis passing by had fares to/from the airport. It is a short walk to the Tube. There are a number of tourist-type hotels in the neighborhood, the Marriott is probably the nicest hotel in that area. IIRC, the rooms were slightly larger than Grosvenor, service was very good, and there was a decent restaurant on-premises. Health club and lounge available (they invited me to use the health club to shower and change when I arrived one morning with a lunch meeting scheduled and the room not yet ready).

Even though my tastes in London run to smaller, unique hotels like the Stafford, I would stay at either one again without hesitation. (Side note: the Kensington property was a short walk to my former employer's London offices, so it became my hotel of choice for business trips to London). For a tourist, the Grosvenor facility is probably the one I'd choose.