It really depends on you, and what you are looking for. If you are just looking to pick up some general knowledge, join namepros and dig in. If you already know the business and are looking for a more professional environment, go ahead and join DNforum.
might as well join as many forums as you can for a more diverse perspective. Be sure to run a google search for "dnforum coupons" or dnforum 50% off" (or similar) before you sign up - it worked for me and saved me some money!
I am a member at both. Namepros I don't get much of a response to many of threads. If you have a good portfolio of domain names, I would go with DNForum. I am pretty sure most people at DNForum are probably a member here and Namepros as well.
I would recommend DNForum, but if your don't like paying namespros is the way to go. I almost forgot you can get like 50 percent off or something basically go to the purchasing screen and then close the web browser and a chat box will appear with a code of some sort.
I think Namepros have some excellent features in place. The chat is always active, the auctions are good, I like their NP system and there shop setup. I think its a superior forum in terms of community, but if you have good domains that are worth $$$ you would be mad not to be in DNF. Like another poster has mentioned, most people register at both, myself included.