Seconding Phoenix Command... All the reasons given by Snarf are quite true.
Aliens Adventure Game (Leading Edge, out of print) is a better rendition of the engine, with slightly lower fidelity, but much faster playtime.
I'll point out Hero System as a very solid universal engine with strong tactical play. It also can be simple to upgrade gear... let the signature gear be built as character point stuff, while the issue is cash value (points for ballance purposes, perhaps) The narrative factors are in character gen, as is most of the complexity - a hallmark of many of the games of it's starting era. It can handle far more granularity than Savage Worlds,
Twilight 2000 4e can readily be played highly tactically, but the X-com type combat will need Urban Operations as well as core.
Twilight 2000 2.x and Dark Conspiracy: these two are the same game engine, but with different careers and equiment. Aliens are right up the setting of Dark Conspiracy. The GDW pdfs are on Drive Thru, and the CDs/thumb drives from FarFuture.net. (If you want thumb drive, you need to mention that in the order.) Likewise, Traveller: The New Era is the same system, mostly, but isn't quite the same. Uses 2m gridded combat by default, phased actions, and more "modern" weapons than you should need... they're current to about 1990.
GURPS While I dislike it, it's well suited, but the combats are unrealistically fast, and generally, it doesn't reward tactical play as strongly as Hero or AAG.