While the graphics of JA2 are far superior to the ones of JA1 I would say that JA1 is definetly worth it.
Actually when JA2 was released it was a disappointment for me at first, because I was missing some stuff from JA1.
JA1 has enemies in all sectors, which makes it a longer fight for victory. It also has a different and imho better ressource management system. You need to decide where to put up guards, where to put harvesting guys and so on. After each day you get somewhat of a recap of what happened this day, which always feels awesome.
There are also sectors where you get special advantages when you sneak. There might be a hidden stache which explodes once the enemies spotted you - no such thing in JA2.
You also have microphones within the game where you can listen to the talking of your enemies. Never had any advantage from that, but it was fun anyways.
In addition it's also fun to get some of the backstory. While the games have no "direct" story which follows up, they have a timeline and all of the mercs have their background story which was worked at in each game.
Maybe you remember lynx and buzz. In the first part Lynx is solo, in DG he has a relationship with Buzz and in JA2 they hate each other. And that goes for tons of characters, like Larry with his drug problems and so on. In JA2 you have got a whole hall of fame of old characters. Ever wondered who these guys really were?
JA1 also had native characters, like you have Dimitri in JA2. Remember Hamous from JA2? Well, he appeared as native in JA1, and became a Merc in JA DG.
If you love the story and the backgrounds of JA, you should check out Jagged Alliance 1. It also has great gameplay of course.
JA DG however is mostly aimed for multiplayer and I would not necessarily recommend it if you only play it singleplayer.