Shutterstock seems to make the most money - especially for vector art
I know a couple of photographers that are on the site - and they'd back me up on that - it's not just that the clients seem to like vector art more, but that it's easier to get images approved on shutterstock if they're art, than photographs. There's so much stuff you have to watch for in a photograph (such as pixelation, blur, or noise) - but I've hardly had any problems with my illustrations. The biggest problem is that if you work in vector art (Adobe Illustrator) then you have to back save everything as an Illustrator 8 eps. That's no problem, except if you're using filters or effects or things that weren't compatible with version 8. But since I found that out - I usually just avoid those things (like gradient mesh and I don't go too crazy with transparencies).
Anyhow - here's a breakdown...as far as it's worked for me:
Shutterstock - 95 % of microstock earnings have come from Shutterstock
Dreamstime - 3 % - this is a hard one to judge though - they take a little bit more time to accept any images (shutterstock approves images in about 2 days, if not sooner - whereas Dreamstime takes about a week AND then another day or so to upload into the search engines...so while the images might have been created at the same time, they're still finding their feet on Dreamstime) - if you wanna check out this site - http://www.dreamstime.com/res1220499-dreamstime - that's my referral number...