If you spend $750 for the gears are you still going to want to update to disk brakes, new lines and a dual master? If so, the budget comparison is faulty.
I don't think you'd ever notice the difference between 3.38 and 3.42, but posi can be real nice if you ever find yourself on a slick surface.
If it were me I'd swap rears, but maybe not put disks on the new rear. Actually, I did swap rears, and put disks on the front but not the rear.
