That's a hot topic right now. Hope it won't get too bad. ^^
Well for starters I do not see a real competition in D2X-XL. Not because I think one of the programs is superior to the other but because of two factors:
1) I think both ports have different goals.
DXX-Rebirth follows the KISS philosophy. Keep it Simple, Sexy. Being close to the original feel and so forth.
See here: http://www.dxx-rebirth.com/philosophy/
D2X-XL on the other hand adds many new features. New graphical effects like High Resolution textures, Visual effects like shadows, smoke, shockwave effects (which look VERY awesome I think). And it uses new Gameplay features like Speed tunnels you see in Descent 3, and other additions/changes compared to the original game. Keep in mind I am not aware of the complete feature-set so don't hurt me if I said something wrong now in terms of Gameplay changes. And as far as I know, most if not all additions are optional like in Rebirth, too.
(NOTE: D2X-Rebirth can also play Descent 1 misisons but compared to XL this feature is undeveloped due to the fact that we have D1X-Rebirth. Also due to this fact I'd like to develop this feature different so Descent 1 missions inside D2X-Rebirth could be played WITH some unique features of Descent 2 so the gaming experience is a whole fresh one. But that's still a bit away.)
So for me personal XL does go into a way I personally do not like. However PLEASE consider this is just an OPINION - not critique or "flame".
I will not give details on "why" becuase I feel that no matter how often I stress that this is just my opinion, it might be considered as flame.
2) We cannot be in a competition since we do not participate in a market, we do "not need to fight over our users" to ensure our income, or something like that. Mainly I do Rebirth for fun. I think it's a valid alternative and there are a few active users here in the forum which actually use it. It's an alternative like FireFox or Chrome in terms of web broswers for example. Some might argue Rebirth just "is not necessary". Some might disagree. I think there is no wrong or right answer to what's the "right" or "best" port. It's a matter of preference - and highly subjective.
Still DXX-Rebirth is my hobby. I'd work on it no matter what. Simply because I love the games and I want to create my personal "vision" of Descent and Descent 2.
And since I do it anyways - why not share it with others? That simple.