Here is how I decide this very important question:
1) what processor converts better on what country? (ie CCBill might be better on Canadian traffic than Epoch)
2) what processor is likely to be around for a few more years? (A very hard question, but a very important one)
3) What processor offers the most services? (what types of cards etc and what languages they support)
4) What processor has their join pages up and workng the most? (another crazy question, but if you poll each processors join pages from around the world like we do, you would be surprised with the results)
and
5) How hard is it to integrate the processors systems into your sites? (take my word for it, some processors are plain painful)
Now this isnt the entire list, but it is a good start
BTW - I obviuosly disagree with gothweb on the multiple processor point, and I wholly agree with what KK said. YOu need more than one processor guys, even if its just in case one of them goes broke