I'm not sure why you have both on your team, but I'd probably play Nevin. It depends on your park, though, since that will affect Batista's HRs and how big the AVG gap is between them. I see it as pretty close to a wash.
A- and C+ players aren't going to get huge numbers of + or - plays. Maybe a swing of 5 between them. And maybe Nevin makes 20 more errors, so that's 25 negative plays. Since it's 3B, a few might be more than 1 base, but on average that will probably cost you 20-25 runs. There may also be some small hit to your ability to turn DPs but I can't measure it.
Nevin will get on base about 43 more times in 650 PAs, and slug for about 16 more bases. What's that worth? Probably 15-20 runs, a little less if the difference is mostly walks and a little more if it's a lot of hits.
If you can get Batista in for defense in the late innings, that lessens the impact of Nevin's glove. And he's not a terrible hitter if he has to come to the plate, though you do lose him as a PH for someone else.