1. What is AT in this case? Is it AMT?
AT is the torque convertor, it shifts between 6 gears, in Kia selto diesel AT variant. I personally drive the Jeep Grand Cherokee 5.7Lt V8 which has the AT torque convertor 8 speed transmission - I love this - its very responsive and quick to shift gears. But I have to personally try out the seltos diesel to compare performance of the 6 speed AT
2. As for the CVT, I have heard people say the CVT variant of Seltos, the petrol IVT is much comfortable to drive. Its smoother, and very well refined as well. But one should test to comment on if thats true, check out if the typical rubber band effect is bothering you a lot.
3. Other than these two facts, one main differentiating factor between the two medels is, the diesel AT comes in HTK+ variant, and Petrol IVT comes in HTX variant.
HTX have tons of premium upgrades in comparison to the HTK+
As a guy who loves features I am automatically attracted toward the HTX, but I have to check out if the Diesel AT torque convertor transmission outsmart the offers from HTX petrol CVT
Hope this helps