To be honest, I had trouble with elance when I started out. rentacoder has a structured 'interview' approach, which forces you to think about your project a bit more. Elance is more just like writing a post on a forum. Faster for experienced users, dangerous for newbies IMO. I also don't like the way elance's escrow system works. I've just had better outcomes and experiences on rentacoder.
On that topic, ALWAYS USE ESCROW and under no circumstances release payment until you're happy that the work is finished! Don't be sold into releasing it early for *any* reason
Cheers