Most stores carry primarily the ocellaris, they are less expensive, I've heard less aggressive, hardier and easier to breed in captivity. I've only kept ocellaris. They are fun, cute little guys. 2 should be fine in a 20 gallon.
What is your plan for filtration? Fowlr
Also, I highly recommend rodi
water and a ato (auto topoff) I didn't have one for the first year but it makes life much easier.
Here's my 40 gallon mixed reef, I'm planning to move some stuff around and turn my 125 g over to saltwater this winter.