I still occasionally play XWA (used to be part of DS back when there was a reasonable online community, NRN etc, before it moved from MS Zone to Errant). Tie Fighter was the game that first got me into PCs, absolutely loved that game, was one of the first games I got, and probably still one I have the fondest memories of, Half Life is but a spec in the eye compared to this, and I learnt a lot about PCs at the time because of it....DAMN YOU EMS MEMORY!!!!
XvT lasted better I think simply because there was a larger userbase because it came out a lot earlier, had initially better support, and was really designed around MP.
Really wish they'd go and do a modern one; I'm pretty sure it'd sell reasonably, even if it was just to enthusiasts; XWA made it to the LA Gold (I think it's gold) series of classic games so it can;t have sold that badly!