A lot of people would claim that the violin is harder because it has no frets. I disagree. Both guitar and violin are hard to learn WELL.
The biggest thing in my opinion that makes guitar difficult is playing clean and getting really good at articulation.. There are just so many different styles and techniques with the guitar than it takes a LONG time to master it.
The biggest thing with violin/viola is not that they're somehow any harder to play technique wise, in fact since they're tuned to 5th, small, and have only 4 strings, I'd argue that they're much easier to navigate than the guitar. The problem with learning violin/viola is that they have no frets, and so unlike guitar, you need a very very good ear to be able to get the right intonation. Having perfect pitch is great for violin/viola, but not necessary. But having a great ear is necessary, and get be trained. If you've got a really good ear and can get good intonation, I think a guitarist shouldn't have a whole lot of trouble learning the violin/viola.
(My source on this is that I used to play violin a long time ago when I was a kid; never got that good, but good enough to understand how it works and why it may be hard for some people)