Depends on your experience
Depends on your experience and if you have ever swapped out rear end gears before.
Lets look at costs new gears vs a used rear end.
Both cost about the same in $$$, and the real savings is in time and the amount of expertise required:
A junk yard rear end can be bought complete drum to drum. To remove the old rear end the steps are simple and easy with no special tools required.
Unhook the driveshaft... unhook the brake lines & emergency brake cable... undo the two lower shock bolts... undo the spring perches... drag out the rear end. rinse & repeat with new rear end.
Setting up a new rear end gear set requires removal of the rear end from the car (to make it easy), removal of the gland nut holding the pinion gear to the drive yoke (torqued down 180+ lbs)... pinion gear pulled out of the case... all this after you have drained the oil, pulled the c-clips.. the axles... spider gears + bearings + shims (keep them in order!).
Etched in the new pinion gear is a clearance that you must use when setting the clearance between the ring & pinion gear... use shims for that. Then check to see if you need to shim the pinion gear closer to the ring gear so that the gears mesh correctly in drive & coast modes.
Baby the new gears by driving a moderate speeds for the first 1/2 hour of operation... allow to cool then drive for reals. I have left stuff out... like adding the fluid, but the real argument here is that it is faster to just change out the rear end if you want it done fast (4 hrs at most).
Once you decide to swap rear ends, you job is to find the best one out there, at the best price, the most complete drum to drum with brake lines still attached... with the gear set you want.
Bonus points for you if you snag a posi unit! You will be glad you did.