I have my portfolio at several agencies. As far as I can tell after one year and about 1000 illustrations online,
Shutterstock is doing best for me and increasing month by month. Dreamstime was doing well, but new images are doing really bad since two or three months. Fotolia likes my vectors and accepts everything, but customers do not seem to like vectors a lot, my guess: as there is no zoom function for eps or svg, but for jpg customers tend to buy jpg. Stockxpert is doing better every month but unforunately are giving up the cooperation with jupiterimages and photo com, IStock is for sure doing well, but as they don't like my stuff, I can not tell. I would like Fotolia to catch up, they were doing best when i started but sales went down in the last weeks, hopefully changing in the future. Then there are several others but not good enough to mention.
Is that more or less your experience, or do you have some other opinions.
Cheers