As they say - the table never lies - Wimbledon, 4th, will be strong and physical and also full of advice for you. Wimbledon got promoted last year via the playoffs. Colfes haven't won a game this season and clearly struggling, having been L6 for quite a few years. I haven't done either of them this year but have seen Wimbledon play a couple of times. Friendly Club, but no referee facilities - if you are the only home game you will have your own changing room; if not, you will have to share with 2nds/3rds etc.
Never having had to stay in any of these, here are a few closish to Wimbledon RFC :
http://www.thewimbledonbedandbreakfast.com/ - 5 min walk away from rugby club
http://www.cannizarohouse.com/ - posh place on Wimbledon Common (30min walk to RFC or 5 min taxi)
http://www.justinjameshotel.com/ - 5 min walk from Wimbledon station (BR/tube)/10 min bus to RFC
http://www.wimbledonhotel.com/index.html - pretty much as above
If you are coming by car, then the choice would be wider - Kingston/Putney/Wandsworth etc.
The nearest station for Wimbledon RFC is Raynes Park. If you want the full directions, let me know.