There are two sides to this.
On the one hand: Go with viola. It's a bigger instrument than violin, has a deep, rich, and mellow sound distinct from the violin--gives you that "stringy" sound without that creepy-crickets kinda sound.
On the other hand: Violin reads treble clef; Cello reads bass clef. Viola reads ALTO clef, or movable C-clef on the middle line (i.e. middle C is 3rd staff line). If you're coming from the background of a wind instrument, other string instrument, or keyboard (piano), then reading alto-clef is going to be a little different. If you're already, say, a trombone player or a bassoon player, you might not have quite as much trouble if you're used to reading tenor clef and can make the switch to alto clef--they're both similar, just a third apart.
The other difficulty is the size of the instrument. The viola is one of those weird, in-between instruments and might cause pain in your left wrist. Viola players typically will complain of pain in their wrist due to an awkward playing position as compared with the smaller viola or the much bigger (floor-resting) cello and bass.
If you don't mind learning alto clef and the possible slight strain on your left wrist, you might do OK with viola. Personally, I learned to do it in college and it didn't bother me much.
Now my problem is when I write music, I'm so used to writing everything in either treble or bass clef that I actually start with my viola parts in treble clef and then change the treble clef to alto clef, an easy cheat with software. I'd be a mess writing viola parts the old-fashioned way!