Would anyone notice the 27 HP and 29 TQ difference??
Same rpm curve?
Simply put, yes.
There are other considerations that should probably also factor in, the design of the engines and the extent that FCA went to make the 6.4 suitable for truck use. Additionally, think about how you may be loading your potential truck up for whatever your ultimate build will be.
As my Power Wagon sits now, semi-dry, i.e. not fully loaded for an extended off-road adventure, she sits at 9,125lb on a Cat Scale. Personally I'am happy that I have the 6.4. Also I'm pleased at the way it tows my 6,600lb. boat here in AZ. So given the choice, there is no doubt in my mind. Additionally, a friend of mine has a '06 PW with the 5.7, has voiced his desire to have the 6.4 in his after multiple experiences riding in mine. I know that is anecdotal.
Mileage is going to be a wash IMHO. Around town here in Phoenix, I consistently get 13.3 indicated which, pencil vs. computer estimate is real close, hedging towards underestimation. Highway, 14.2 - 14.4 if not exceeding 75 mph.
Reliablity, I've had two warrenty issues (53k on the clock currently). Passenger side center air director for HVAC was broken on delivery and at 28k a solenoid failed in the EVAP system. Overall, extremely pleased both on and off-road, it's an equal match to my 80 series other than perhaps width.
Hope this helps.