I have been associated with Elance for a long time and I did try odesk but some how it did not work for me,especially the application part which has to be downloaded for your client to check your activity.
I've hired all my best team players from ODesk. They screen their members well, even ask them to send in a scanned copy of their IDs. They also have a tool where the employer will be able to see screenshots of his team's computers while they are working.
Odesk is the better option for me, you need fast internet speed, the work is loaded in software which we have to start before working for clients.. All smooth transactions..
Elance is new and its not perfect than Odesk...
We are on oDesk. It has been wonderful tool giving confidence to the buyer and the freelancer. Buyers can see the activity of each worker with screenshots from his daily work. Good confidence it gives and payments are good AFAIK.
All the best in your selection.
That's true oDesk is good selection I think, I also prefer.