There's also a difference between classical acoustic guitars and ones that are not... classical guitars tend to have a wider spacing between the strings for better control over plucking, etc, and don't usually use metal strings. 'Regular' acoustic guitars are spaced basically the same way electric guitars are and use metal strings as well (like electric guitars).
I would recommend picking a guitar depending on the type of music you want to play. There are styles that you can only really play on acoustic (classical is hard on electric, as is flamenco, but any metal, crazy rock guitar solos, etc, are not playable on an acoustic and have it sound anything like it should). For getting the basics the acoustic might be better, but I started on electric (my dad also had an acoustic at home so I fiddled around with both) and I can switch off between the two without issues in terms of playing... the sound is of course completely different and used for different things.

Though it has to be said that what can be done and how accoustic sounds is evolving. See (among others) Andy McKee (I saw him and two others live recently \o/):
http://www.youtube.com/watch?v=nmE3QaGetn4
http://www.youtube.com/watch?v=Ddn4MGaS3N4
http://www.youtube.com/watch?v=RlDdzNuJto4
Or Don Ross:
http://www.youtube.com/watch?v=ip111lXjDSs
And and and ... :D
http://www.youtube.com/watch?v=tGemE401r-M
http://www.youtube.com/watch?v=b1GZC0GQKOA
Or well ... pretty much anyone with candyrat.
http://www.youtube.com/candyrat

Clarinet. I played for four months, before I realized that my fingers couldn't reach as far as they needed to for some of the more advanced notes.

I've always loved the sound of the Electric Violin.
Alternatively: You can't do much worse than a Lagerphone!

I already play that.

I play guitar and piano, but if I picked up another instrument I'd either go drums or violin.

How about the Theremin?
Super Mario Theme On A Theremin

if you get a violin, learn some fiddle songs
i've played violin/fiddle for over a decade and i would have stopped a long, long time ago if i hadn't found some fun songs to play
violin music can be really boring. unless you're really into classical music.

Heh, I'm actually a professional piper. I make a decent chunk of my living teaching bagpipes.
I also went to Berklee for upright and electric bass. If you want to play a lot with other people, pick up the bass. If you want to pick up the guitar, get an electric first, it's much easier to play.

I vote for the spoons. Learning how to shred on the spoons is an art and there are never enough spoon players in the world.

Play the Theremin. You'll feel original.

Ohhh that's the one

Tenor sax. I always wanted to play that, so if you go ahead and do it, I can live vicariously though your successes.