I like DP more because of the freedom no fees for listing etc. SP is ok but hate the damn pops and ads everywhare.
When I compare sitepoint with DP, I still think DP is much more fun and entertaining because there are more members over here.
If the website you want to sell is quality then you'll get people who will make serious offers, rather then try to make lowball offers. However, they charge $10 - $20 to list a website, but its worth it imo.