Secretly I've never actually played the original X-Com even though this doesn't seem like a good idea from what I've heard of it. I REALLY enjoyed Jagged Alliance 2. Would I enjoy the original X-Com too?
I think it's worth a try. It's... in a similar genre as JA2, but the games are still pretty different. You don't worry about losing people so much in X-Com. (Or at least, I don't. Some people reload after every death.) In JA2 though, you did have a somewhat limited and very fixed number of mercenaries you could recruit. And some of them cost enough that it was prohibitive to have to rely on them too much. In X-Com, your number of potential recruits is basically limited only by your funds with none of them costing more than any other.
Jagged Alliance, to me, feels more like an RPG than a Strategy/Tactics game. Every person has their own story and personality and you do feel bad about losing them in almost every case. (Screw you, Biff.)
In X-Com, it's more like chess. Yeah, your guys are going to level up, and get better, and you may even grow attached to your higher ranked players (Queens, knights, rooks, bishops, etc.) But everyone starts out as a pawn. And unless you're playing it more as a roleplaying experience than a strategy game, you're not going to care about losing a one, two, or a dozen pawns. At least not beyond how hard it hits your bank account.
This is reflected in the settings though.
Jagged Alliance, you have mercenaries, most with better things to do beyond fight for a small country nobody's probably heard of. They're just doing it for the paycheck. They have hopes, dreams, feelings. (Screw you, Biff.) 99% of them would much rather get out of it alive rather than complete the mission.
In X-Com, the fate of the world is in your hands, so what if a couple hundred people have to die to save it. These people are volunteer soldiers, and supposedly the best of the best(Perhaps the best that volunteered, as some of them seem incapable of hitting a barn, literally.) and are prepared to meet their fate. (At least up until the point where their commander gets killed right in front of them, people start getting mind controlled and a blaster bomb takes out half the squad. Also, screw you, Biff.)
In X-Com, if these people don't do their jobs, there won't be a world left living on for them.
In Jagged Alliance, if these people don't do their jobs, oh well, one more ruthless dictator in the world won't really change the status quo that much.
To me, those are the main differences. Everything else can be equated, perhaps not perfectly, but at least close enough to call them similar games. JA2 has a deeper story, but X-COM is more purely a war game.