Flip a coin basically.
Ram is a proven quality platform with what is currently the best >6 speed trans on the market, until the 10 sped proves itself. It's going on 7 years being used in virtually evey Fca application through the hellcat and demon. And longer and millions more in Europe.
7.3 10 speed should beat it up a hill based on the numbers and indications are it's not problematic but it is first year, so there's that.
6.6 gasser should be a nice engine. Hampered by a 6 sped and an ugly truck u less you get a GMC. That only fixes the looks not the trans. 6.0 GM HDs were solidly in last place prior to the upgrade and still are. Not by a huge amount and better than your old 5.7 Hemi by a good margin, but it's not like they cost less because they're slower!