Usually cheaper and easier. You don't have to buy a complete eng.,wiring harness,trying to mount a crank sensor on the bell. Also alot of guys are running a 4.0 with a carb. I think most that have done this would have gone for a swap, it's just this was easier on the pocket.
My$.02

http://home1.gte.net/rythem/davescj7.htm
rythem@gte.net
"Get your finger outta there"

 

I think the two biggies are:

4.0 head gives much better breathing (larger valves, smooth flowing ports)

4.2 block has longer stroke. This is better for overall displacement, but more importantly better in torque.

The main disadvantage is the possibility of having a bad short block.

The 4.0 head and MPI combo will usually run more $$ than a straight 4.0 swap, but all new EFI components and head work are new as opposed to having some miles.



-raymo
4.6 Stroker Scrambler