The 6.4 tows better, more about the TQ than the HP, a 5.7 downshifts a lot without a trailer, my 6.4 (not exactly stock) pulls way better than a 5.7. I actually buy 5.7 trucks with bad engines and stick 6.4s in with a simple upgrade recipe that is much better than even a stock 6.4.

If your willing to build a truck up the best combo seems to me to use a 6.4 truck short block, 6.4 SRT8 injectors with 5.7 heads, intake cam... as the 5.7s make more TQ per CID than the 6.4. This also gives you more compression than a regular 6.4 truck and is a beast for towing, I currently have this exact combo in my 2011 4x4 4 door long bed and towed an identicle configuration truck on a heavy 20ft tilt-deck trailer and even going up a couple big mountain grades only un-locked the TQ converter and did not downshift. I got 11.5 MPG towing that truck. It pulls even better than an actual stock 6.4 truck and cost about 1/2 what one of those trucks would cost. The real 6.4 trucks use EGR witch I hate, the 5.7s don't have it.

I did one with a 6.4 SRT8 engine and it pulled even better but it liked to run much higher RPM also, this current one is much more satisfying when pulling.