This is a tough one. I have done it both ways when I had my behringer mixer. The pre's kinda suck, but I can only assume the a/d on the microtrack sucks too. having another a/d such as an sbm-1, ua5, or a graham patten would make this a no brainer. I guess I would use the v3 on the mic source only if it will be predominant in the mix.
I.E. if I was stage micing an amplified band, or micing fob I would use the v3 as a pre and take the a/d from the mt. If I was micing stage corners for a little ambient room fill I would use the v3 as the a/d.
Keep in mind too, that without delay you should stay within 20' of the PA for best results. If you are going to be 20-30' back I would heavily favor one source or the other, and 30-45' I would use just a touch of mic, jump it between songs for good crowd reation (while monitoring), and pray you are getting a nice full-range stereo board feed (you may want to bump up the bass eq ~3-5db unless it is a really big room or a really puny PA). In the case of being far back from the PA use the v3 as A/D.
Good luck with the matrix!!! I love matrix recordings!
My .02
Matt