Pretty much the topic title. Also, even though I have UB, should I get the others as well?

Jagged Alliance 2: Unfinished Business (also known as JA 2.5 and JA2:UB) is a more combat focused game, with more or less sector fight after sector fight, taking place in a different setting than Jagged Alliance 2 (aka JA2). It does support user made campaigns, and if you look around you should be able to find a few (SOG '69 is one of the highly praised).
Jagged Alliance 2: Wildfire (aka JA2:WF) is more or less JA2 with a few more mercs, a few enhancements, and the difficulty cranked up. The maps are different from normal JA2, the miners won't just agree to turn over the income to you, you will need higher leadership and town loyalty, and there are a few more enemies on the maps. On the upside, you can have increased resolution, and a max team size of 10 instead of 6.

On the other hand, JA2 1.13 supports both UB campaigns and WF maps, along with higher resolutions, increased team sizes, more mercs and a ton of other enhancements. Grab JA2, then drop by The Bear's Pit and grab either the stable release or the newest one, and be prepared to get stomped into the ground.

Should you have any questions about JA2, don't hesitate to ask.

IIRC Wildfire was the very first JA2 mod which was later officially released.
If you never played JA2, you might want to play JA2 first before upgrading to 1.13.

And even after finishing JA2 vanilla you'll still get your arse kicked with 1.13.... in a good way :D

Thank you for the responses everyone!

Is there any way to change the screen resolution manually?
I played JA2 under wine in linux and had no problems with it; JA2:UB, on the other hand, stays at a very low resolution.

From what I recall, both JA2 and JA2:UB only support 640x480 resolutions (talking about the unmodded games). Don't recall them having an option to change that resolution.