Thought I should hop back in here to say what I've bought and my experiences:
I bought the Behringer UMC404HD, and I am very happy with it. It works perfectly for instruments like guitars and my theremin, even with a specific setting for them. It's simple to set up, having just plugged in and been detected as an ALSA device, and then worked perfectly fine within JACK. It has plenty of features to keep me entertained, and while I might not be using all the channels to their full potential, not having to so much dick around with physical cables to play a different instrument is definitely nice. Looking neat doesn't hurt it either.
I'd offer some examples, but I am a very beginner player in everything, and my examples aren't great ones of how good it sounds. I'll have something some day when I improve, so if you for whatever reason /need/ to know, hmu.
To be more specific of the sound quality, it sound excellent for my ears. I run it through a studio monitor, which isn't the best for hearing the minute details, but it's well above my PC's sound card, which would have still been acceptable. It works very well with my Mac Mini DAW running Arch Linux, and I've done all sorts of neat things, like DSP guitar effects, analog synth, recordings, playback, etc. This is truly now the DAW of my hobbyist dreams, and I've had loads of fun with it (which was the point).
Thanks everyone for suggesting different options, honestly all of the suggestions would probably have worked fine enough, but I liked the Behringer for looks and name.