I am of the impression that Epoch will simply not accept new clients (or, for that matter, existing clients) who do not process Visa. However, as others have said, 60 to 70% of US-based purchases are with Visa rather than Mastercard, though I suspect many surfers probably have both cards.

For a variety of reasons, I really believe CCBill is the way to go if you're after a single processor. Verotel seems to be very solid and reliable, but I've always heard spotty things about support and being able to reach people. Segpay has a history of processing for questionable sites, and all the others out there are either new or smaller.

It won't hurt to dip your toes in the water and do mastercard only... and don't forget euro debit, also offered by CCBill; a very large portion of European customers don't use credit cards for this sort of purchase. But if you start getting any sort of traffic or sales, or expect to get affiliates, you'll need to bite the bullet and get Visa approval.