I think if you are considering EFI I honestly dont think it would be that much different in cost to do the 5.7 in comparison to the 360. I haven't priced out both fully but looking at the cost of the motors its dependent on if you are going crate motor vs pull out. You can probably get a pull out 360 cheaper, a 5.7L hemi pull out is cheaper than a crate 360, a crate 360 is a little cheaper than a 5.7 etc.

EFI electronics wise it is about the same to do either engine possibly a little more expensive to do the 5.7 because there are fewer of them around but if you are looking for after market wiring and not a junkyard pull out it is closer to even.

In terms of putting the car together and doing it yourself it is easier to go carb'ed if you have the mechanical ability. EFI is going to be the same with either engine, with exception of the drive by wire issue. Yes the distributor etc makes the magnum motor a little easier to wire but if its a factory pull out its about even.

Fuel mileage wise it looks like the hemi's do a couple miles to the gallon better in a lighter vehicle (not trucks) but a well tuned carb motor can get great mileage as well if the driver doesnt stand on the gas all day.

In other words everyone has an opinion and you need to decide what you want to do and what you are capable of doing/learning because that is all that is going to matter in the end.