There are many differences... off the top of my head...
The 87-93 cars were rated at 225 hp stock, the earlier cars were less by 20-50 hp depending on the year. 80-81 models came with a downsized engine and are generally great to avoid.
The tail light panel changed at some point in the early 80's.
Prior to 1986 all Mustangs were equipped with the 7.5 inch axle. The 8.8 Axle became standard on the V8 cars in 1986. It is a significant improvement in durability.
The K-member is narrower on the 79-86 model cars (it is the same as the 87-93 2.3l cars however)
Front Brakes, although still puny, got a size increase in 1987.
The interior was completely redesigned and became far more modern.
Ultimately, due to backward compatibility and the SN95 parts being superior and swappable to virtually any Fox Mustang, you should really pick the car you want based on the style that you like. If you like the 4 eye cars, they can be built to be just as good as an 87-93 car in terms of performance.
Oh, as for fuel economy... they all suck.