AndresR reported on another forum that Fotolia has been his #1 earner 2 months in a row. My data sample is much smaller, but Shutterstock passed iStock in earnings in January and February.
I like iStock despite the miserably low commission. But they represent only 25% of my earnings these days, so I would not consider exclusivity. Also, microstock is so young. Who knows what new major players will emerge this year or next, or who might flame out.