I have -12 offset. Love it. I would've gone with -24 20x12 but there was no way I could fit them with my lift and 37s... I have pics in my build thread. I love my look and you'll be happy with -12.
The only MT I would go with is a Toyo MT because it'll get you some decent mileage. I got 35k on the 2500 Cummins and that was with poor rotation. I am expecting 40 out of the set I have now. But other than that go AT.
I personally don't like AT on big lifted trucks but that is just me. I think you'll regret it. I do a lot of highway driving so I went with Toyo MT and they have always lasted me. Other MTs can barely seem to get half what my Toyos get.
Good luck and if you have any questions let us know.