Speaking of guitars, think of how you are going to get to the right signal levels. If you use an external preamp anyway, a good line level IO should do.
-- AcdNrg
Not so much. See here: http://www.ovnilab.com/articles/linelevel.shtml But the TL;DR there is that instrument-level signals require much more gain than incoming line-level signals. For one thing, they're typically -24 dB down from pro line-level. Also, the impedance on that input will need to be different from a typical line-level's input, and that can cause signal loss, or if the input preamp doesn't have the necessary gain, you'll wind up amplifying a lot of noise along with your desired signal. Also, with an external input, I think it's important to have something that not only amplifies, but which also has an envelope follower, such as the Doepfer A-119. Being able to modulate with the envelope follower is a big key to getting instruments such as guitars to be fully-functional controllers in addition to signal sources. The A-119 also has a comparator to extract gates from signal levels, and this comes in handy for firing off envelopes, LFO resets, etc.