A quandary

I'd say go with the light weight wheels, reducing un-sprung weight and weight in general is always a good idea.

I'm doing the mopar spindle and disk swap from the newer A bodies to get my 5 on 4.5 bolt pattern, might save a few bones over the mustang stuff. Maybe enough to do another mod?
The mustang brake rotor swap as mentioned here:
http://www.forabodiesonly.com/mopar/showthread.php?t=20544
Then I keep my 4 piston calipers, and don't have to search all over for 73+ A-body cars.

Another benefit of different wheels is I can get some more backspacing and run an inch wider tire.