There's a few good points in this thread.
Reddit is a relatively mainstream website with a large userbase, so it's great for discussions of just about anything if you find the relevant subreddit. Circle jerks are relatively common depending on the subreddit, but it's easy to just unsub from them and be on your way (also, sort comments by best and not by hottest, or whatever the shitty default setting is). The strength of the site lies within its upvoting system which, contrary to what Blanko said, generally promotes varied discussion.
4chan has a much smaller user base. Most of the larger boards that made it famous contain a lot of shit now, especially /b/, but there are gems among the shit. There's good discussion on the various boards from time to time, but the format has always annoyed me.
Plenty of people frequent both, even if some people think one site is cooler than the other.