Radial T/A vs. Comp T/A ZR

[84LX89GT]

A BFG radial T/A is an S-speed rated tire with a softer sidewall and worse handling characteristics than the Z rated Comp T/A you're looking at. In fact, auto manufacturers recommend not using a tire with a lower speed rating that stock which in the case of a fox mustang GT, is a V-speed rated tire ('87-'93). An H-rated tire would be good enough for most performance use, but an S-speed rated tire is more of a higher mileage, lower cost, lower performance type driving tire.
Also what size rim are you using, is it a 15, 16 or 17 inch rim?
I would definately go with the comp T/A if those are your options. It's a good wearing/handling tire so it should be good for sprited driving.