Just picked up my 2015 3500 with the Aisin transmission. I had the same debate on which one to get. The dealer made it pretty easy when priced out between the two trucks it was only a $1700 difference.
Cummins is the most proven engine but the worst truck. Ford is still proving there new set up but the best truck.
Chevy has proven the D-max, descent power and a good truck. Happy medium would be a Chevy... and I don't even like chevy.
I get that bud. I'm just saying for reliability reasons go with an auto. You are asking us how reliable these transmissions are etc... Well I can guarantee you they are most reliable stock. So for the best power reliability stick with a stock auto.