Alternity is sweet. I love the Mechaluses.
I say you split the difference and play Alternity.
GURPS vs AD&D -
I generally do not buy any system that I don't play in often. Which doesn't say much as I have been playing games for over 20 years. I have a mostly full set of AD&D 1ed (about 10 books). I have three books from AD&D 2nd Ed, and none from following editions. GURPS -have about 15 or so books 1st Ed, core books (basic, Magic Spells and Psionic powers), and setting books (i.e. Fantasy, Cliffhanger, Supers).
Off and on I have played in long running campaigns in each. AD&D uses many different sided dice. GURPS uses 3d6 dice generally (damage and effects might use additional d6).
I look at the systems as "Background" mechanics, once you get used to it, you can use any system for any genre. GURPS has more setting books for the different genres, but you can adapt AD&D to any genre you want with a little work.
My main criteria is "how often am I going to use this system?" and "am I using this system in any games that I have played more than a few times?" - I haven't played in any long running campaigns of higher editions of AD&D besides 1st ed - and only bought the few books of 2nd Ed because it looked like I was going to be playing in a long running campaign (which only lasted a few weeks). I played in many many GURPS groups for years, but now not so often.
I figure I have spent several hundred dollars worth on AD&D 1st and GURPS 1st, and this was back when the books were under $20 US. Now the core books are costing much more, double and often triple that amount. It is difficult for me to justify spending that level of money on a system that will be coming out with a new edition in a couple years. I'm also an older player, and have to budget my money carefully if I am going to make certain I have money to pay my rent.
My opinion... put your money into a new system rather than one you have an earlier edition for, or put your money into rounding out your books for a system you already have. If you don't have GURPS, buy the core rule book. If you don't have AD&D books buy a core rule book for that. If you already have a core rule books, regardless of edition, expand on those that you have rather than buy the next edition.
This is just my opinion, your mileage may vary.