For the rear end the width is critical for what wheels you want to run. If you're buying a rear end, there's no chance I'd buy one with the standard A-body spring perch location. A 1/2" offset kit from DoctorDiff or
@BergmanAutoCraft would be the way to go, you maximize the tire width without any changes to the wheel tubs.
The 3" spring relocation only makes sense if you're doing a mini-tub as well, without the mini-tub the 1/2" offset gets you just as much room.
Not saying the 9" is a bad deal at all, just know what your plan for wheels is before you place your order.