Based on the stats I would say the '65, it does have a 200lb weight advantage.
I know the older engine is rated at the flywheel, it has around 80-90hp to the rear wheels. I don't know about the '89 engine.
-'65 Mustang coupe 6cyl 3.3L
-2556 lbs
-120hp
-190tq
-'89 Mustang coupe 4cyl 2.3L
-2754 lbs
-90hp
-130tq