On paper, and logically, they should be about the same. Driver makes a big difference, how much extra weight you are hauling around, how good a fuel each car got, and a million other things. But basically, they are rated the same, and weigh nearly the same.
Now to put a personal spin on this. This is just my opinion, and the opinion of several friends. Take that at face value.
89s are going to be faster. It doesn't matter what Ford you buy, if you have 2 different years of the same car, the 89 will run and perform better. I have had many 89s, and it is my favorite year by far. This guy I knew raced Probes and of all the ones he owned, his 2 89s were always the fastest. My 89 old ****** GT ran way better than the others I drove. I have an 89 F250 diesel with a turbo that would outrun and out pull several friends chipped Powerstrokes.