I would strongly suggest getting the upgrade if you decide to stay at the Grosvenor Marriott. The rooms are tiny. My partner and I were bickering after a couple of days because we kept falling all over each other in that small room. We just returned from 2 nights at the Chancery Court and I cannot recommed it enough. Reasons for choosing Chandery over Grosvenor:
1) newer property, 2) far nicer public areas 3) concierge lounge (none at Grosvenor) 4) larger, more modern bathrooms 5) bigger room 6)higher thread-count sheets. The only reason I would go for the Grosvenor is if you absolutely HAD to be within walking distance to Oxford Street. But each hotel is only a short walk to the tube and that puts you anywhere in London in short order. For me it's a no-brainer.
Paul