I think I like everything I'm hearing about the 7.3 and the 10 speed. However I would not compromise on the exhaust break.
Going up the mountain is more about speed and the time it takes to get to the top which is really not important. However coming down the mountain is more about safety and remaining in control.
You don't see runaway truck ramps going up the mountain, But they are present coming down! Maximum exhaust brake ability is a must. Ultimately lack of a true exhaust brake would be a deal breaker for me going with a gasser.