There are a number of threads that discuss the various London Marriotts. Here are a few that might be of interest:

http://www.flyertalk.com/forum/marri...reccomend.html
http://www.flyertalk.com/forum/marri...er-thames.html
http://www.flyertalk.com/forum/marri...ld-elites.html
http://www.flyertalk.com/forum/marri...amily-4-a.html
http://www.flyertalk.com/forum/marri...or-square.html
http://www.flyertalk.com/forum/marri...el-advice.html

The first thread listed is probably the best one (and also the longest) for you to look through. There are pluses and minuses for each of the London hotels, so which one would be best for you really depends on your likes and dislikes.

There are many other threads as well, but hopefully these can give you a start in your searching. Good luck in your search and enjoy your trip.