It's hard to argue the 6.4 vs 6.7 for mountain pulling or full timing. The turbo, e brake and torque alone are hard to beat. If I where full timing I'd have a dually and a bigger 5er.
As far as one pulling away from the other...how fast do you want to run? I'll beat anyone that pulls 55-60 mph no matter what rig they have because I run 65 -70 mph. Anyone running 70-75 mph is going to beat me.
The 6.4 loves to be fed. No doubt about it. But so did my 6.7. I averaged 9 pulling with my 6.7 over 100k miles. I am one of those fortunate guys that never got the bragging rights on mpg on any of my oil burners. Its loves rpm like the 6.7 loves its turbo.
If you're worried about maintanance cost as a budget breaker I would stay home.
As far as major repairs out of waranty the 6.7 is going to eat the cash alot more than the 6.4. For around $2500 you can get a max lifetime waranty that you can't get on the 6.7.
Fuel costs I've always run the lower octane without issue. The smaller tank and a headwind will have you seeking fuel every 150-200 miles.
As far as the OP original question, you Re going to be very satisfied upgrading from the 1/2 ton.
2014 Silverback 29RE, 2014 Ram 2500 CC 4x4 Hemi 6.4 auto w/ airbags, B&W Companion. Saltwater boats and rum like any good pirate.
Loose the 2nd and the rest will follow.