iStockPhoto/ShutterStock are the top 2 players in the field.
ShutterStock recently imposes a 30% US tax withholding for sales from US customers by contributors from non-treaty countries (eg Singapore). If your sales are mainly from non-US customers, then the impact is very small, if the chunk of your sales are from US customers, then the impact is significant.
ShutterStock gives me the most, followed by iStockPhoto and Dreamstime, but to maintain good sales at ShutterStock, you need to upload new photos periodically in order not to get 'buried' deep down.
iStockPhoto is most strict in their photo review. Their model release is also the most stringent.
In terms of %, all the sites have a hierarchy system. The new members get the least. as you progress, % will go higher. For example, at ShutterStock, you start with 25c, then 33c, then 36c (I think). Some sites also offer a different (higher) rate if you are exclusive with them.
hope it helps a bit...