Sigma's internal AFS is called OS, Tamron's is VC.
I have a Sigma 17-70 C with OS and it works great. The internal motor in the D7100 is nice if you have older lenses that need the camera to have a motor for AFS to work.
I've seen a few on craigslist and a few sites for $1000. Nothing less than that or I would have had a hard time not buying it.
Bollocks, I meant HSM for Sigma and PZD for Tamron.