Delta is fine. Economy Comfort is not really worth the money unless your legs don't fit in a normal economy seat.
DUB-ATL has seatback entertainment in the summer months (A330), but in the winter season it's a lottery whether you get a refitted 767 or not.
Atlanta is easy to change plane - the comments above about immigration/security obviously don't apply to Dublin flights. You will have to do immigration/customs/security on the way back from SJO, but at least ATL supports through-checking of international bags (unlike many US airports), so you should not need to collect and recheck.