OpenXCom is just a rewrite of XCom:EU and XCom:TftD, but it is also fully moddable, so you will get access to the FMP, Piratez, Redux, etc. on top of all the other smaller mods created by others. OXC is also a lot more configurable, and has much better UI than the original. I would never go back, even to play a completely vanilla game.