There are quite a few metrics to compare on, such as ease of integration (client libraries, documentation), reliability (downtime, SLAs), pricing (blended vs in

There are quite a few metrics to compare on, such as ease of integration (client libraries, documentation), reliability (downtime, SLAs), pricing (blended vs interchange-plus), customer support (response time might be quick but might take several days to talk to someone technical), countries and reporting. Reporting is actually really important and often overlooked, and especially matters when you get big. Make sure it's easy to reconcile all your payments with your processor statements. You'd be surprised at how often payment processors' accounting is wrong, in their favor.