The biggest problem with violin/viola in my experience, is the bow noise, which can be very harsh, and makes close mic'ing pretty much a non-option. I've been lucky to work with a top-notch violinist on the current project. Audrey White is a full professor at the New England Conservatory- $30,000 viola, $15,000 bow, you get the picture.
We tried a bunch of stuff, but my suggestion is to use the most flat, transparent mic(s) you have. Either one good LD or a pair of SD's will work, and back those suckers off a goodly amount. 2-4 feet was suggested above, but we wound up at more like 6 feet. This means you need a good room, and damned little background noise, or you are screwed. We tried Studio Projects C-4's, Oktava MC012's, B.L.U.E. Kiwi, and C414B-ULS. I think our best tracks were achieved using the Kiwi, backed off about 6 feet, into an Avalon AD2022.
Good luck- a violin is one of the most difficult acoustic instruments to capture accurately, and a pickup will suck for sure.-Richie