The mic you use will display the obvious different sounds inherent to that mic at any given mic placement. You will use what you have whatever that might be.
But, the placement on strings is what it is all about. No matter WHAT mic, you will probably mic it the same way, which is 12"-24" straight above the f holes on the viola. If you happen to have a great sounding room, you can use a omni directional mic pattern and use the proximity of the mic to the viola to control the direct vs reflected sound on the mic.
If you have a not so great room, you might want to mic a bit on the closer side, with a cardiod or even hype cardiod pattern mic. Problem here is that you get more proximity effect from the mic, making swells in the volume harder to contain. So much though depends upon your technique and what not here too! You might be able to overcome the proximity effect with a bit brighter tone, which you can achieve by bowing closer to the bridge. Maybe your viola is already sort of bright sounding like that, and the proximity effect of the cardiod pattern will give the sound a bit more "beef". Who knows!
But, try micing 12-24" straight above the sound holes as a start with whatever mic you have.
Preamps are totally overrated. You can get pretty good results with a Behringer if your instrument has a great tone, and you apply a good mic with mic placement.