The Park Lane Marriott looks to be very posh. However, not sure how they are going to open this month - it still looks pretty much like a shell when you walk by it. The hotel is attached to a small apartment complex and will share some facilities.
I think the location is better than the Renaissance. The hotel is right in the middle of Oxford Street, near Mayfair, and not too far from Knightsbridge.
Can't speak on facilities, etc., but I do walk by the Renaissance on my way to work every day and the facilities, etc. look fantastic - probably a better overall deal than the Park Lane.