It somewhat depends on which 911 you have. Owners go by version number (964, 993, 996, 997) like BMW's E34, E39, E90, etc..
6speedonline is more like E90post. It has a younger membership. More 996/7 owners and plenty of wannabees. But if you are looking for a mature forum, then Rennlist is better, imho. Lot's of PCA instructors post there, and more Porsche owners with a much longer history of ownership. Very polite and informative.
There is also a new forum called Reutterwerk. It was started by the former owner of Rennlist (which was sold to Internet Brands.) It also is more polite and mature.
As somebody already mentioned, the Pelican Parts forum is a tech forum for DIY stuff and maintenance info, etc.. Esp good if you have a pre-996 version. The company also sells parts for both P-cars and BMWs.
p.s., join the PCA. It's very active and there is a lot of mod and tech info available from them. PCA also has a racing series that you can apply for and work your way thru the ranks (like SCCA.)