The 6 speed is tried and true honestly. The 8 speed was a paper weight. The 6 speed can go 200k without a failure if maintained. The thing is to keep the temps down, the thermostat update or bypass is key on these for longevity. I did a updated thermostat and run 140 degrees consistently vs the stock 190 deg.
I do agree with others, the 10 speed is very above and better in every way, alot more refined than the clunk box 8 speed. Dont let a 6 speed rule you out either if your going lower trims. My 15 LTZ 6 has been great after the cooler update... I have heard some guys here say the 8 speed suffered some of the same heat issues and they got alot better after cooler updates also.
Sorry to seem confusing. My preference is as follows after driving everything. 10,6,8 speeds.