My advise to you is: "don`t buy an electric viola"...
Don`t get me wrong... You like your instrument (15"), then keep it, and electrify it. There are excellent devices designed to pick-up the sound from violins that work extremely well for the viola. For ex: you can buy from fishman (www.fishman.com) or L.R. Baggs (www.lrbaggs.com; look for the violin pickups). I personnaly use the L. R. Baggs bridge on my violin.
The "pick-up" option for electrifying an accoustic instrument offer several advantages:
1) It`s much cheaper!
2) If it is well installed, it WILL NOT alter the actual acoustic characteristics of your classical instrument.
3) You carry only one instrument that you know and you love... Especially true in your case (15")
Also, depending on your application (what is your need for an "electric" viola), you may want to go with a microphone. However, if you play with loud amplified bands, or large stages, I do not suggest a mic. This may create problems of feedback, etc.
Good luck, and keep me posted on your decision