I've only stayed at the Marriott Riverside and I too loved it. It looks like a five minute walk from the Mayfair to Chit Lom Skytrain station. It looks like you have to decide whether you want to be in a resort atmosphere or a high rise in the middle of the financial district. The downside of the Riverside is the fifth person sleeps on a cot. If it were me, I'd stay at the Riverside because I think your group will leave with a better feel for Thailand than the mad rush of "downtown" Bangkok. If you're going to be there awhile, consider seeing Phuket. Its really beautiful and much calmer than Bangkok. The JW Phuket is a real gem and you can book last minute flights cheap when you get to Bangkok. Have a great trip.