(GD and ImageMagick only effect the Captcha and Gallery; if you use neither of these then you don't really have to worry about what is right for you as you don't use either of them anyway)
Many users have asked what is better; well, that's up to you to decide really (though IM is more scalable and looks much cleaner :hint:)... Well, I'm going to go a level more and visit Operating System support (with IM s GD).
In Windows, to run IM you MUST open CMD.EXE access to your web server software, which can be a significant security issue; so if you're using Windows as your web server software, I'd recommend you to use GD2.
In Linux (Unix) I'd recommend you to use ImageMagick, items generated through IM look much cleaner than through GD.
Another upside is that if your Web Host does not have support for either GD or IM, you can download a "binary" (precompiled file) from ImageMagick and just upload it to your server, chmod it 755, then link to it and it should work just asif your webhost installed it for you.
One thing to keep in mind is that when you setup ImageMagick for your forums, you must define the path (location on server) to the ImageMagick files (Convert and Mogrify). If you're in Windows you'd need to do double \'s; so it'd be: C:\\WINDOWS\\System32 (assuming that's the location). In Linux you'd just enter the path such as: /usr/bin/convert and /usr/bin/mogrify
When you go to enter the paths for IM, you'll do so in the DB, Paths & URLs section of the UBB Control Panel.