I'll tell you what I know, or at least what I think I know...
Both the years had a 225 hp and 300 ft/lb torque rating from the factory.
The '89 was the first year with the MAF Sensor (this will allow more mods than the speed density system, although you can always convert the '87 to MAF.) Also, in 89, Ford decided to make a cam change that caused less noise, but also lowered engine hp output by about 3hp...
If you take a glance around, you can find a lot of advice about what to do, other than what I'm gonna type.
Subframe connectors improve the handling/takeoff of your car, especially convertibles, so those are always reccomended.
Most of the people here also reccomend 3.73 gears for your rear end, this will give you a very noticable amount of acceleration over stock gears.
Something free you can do is to remove the air silencer box, which I'm sure more than a few of us can guide you through doing, it's really easy, and it gives a less restrictive airflow path into your intake, makes your intake a bit louder (mine was most noticable around 2k rpm,) and people say you get 2 or 3 horses out of it.
Underdrive pulley's, they reduce your accessory speed, freeing up horsepower because it takes less power to turn them, so you get more power to the wheels. Also pretty easy to install, the biggest problem I found was getting the right size belt. This might get you 5-7 horsepower at the wheels.
K&N Filter, less restrictive means more air.
Others can help more, I hope I helped you out a bit, these upgrades weren't put in any particular order, just in the order I thought of them... Have fun with your new pony, wether you choose the 87 or the 89, and welcome back!