5 lug gives more clamping strength for road course and drag racing. Means your less likely to shea off a wheel stud, though I have gotten pretty wild in my Stangs and never had a problem.
Also by switching to 5 lug the options for wheels is much much greater.
I am using a SN95 rear end so I can get the 5 lug and rear discs. The big money outlay now is getting a front SN95 brake setup so I can have all 1996 SN95 brakes.