The 1990 will have 14 more hp than the 1988. They did a little engine work for the 1989 and later models that increased power slightly. This was acheived through lighter rotors, higher compression, and a VDI system. Other than that, the differences between the two are mostly cosmetic.

 

The 90 will have the highly desirable circular tailights in the back, and different looking front grill as well as the aero-mirrors. The interiors have lots of little differences too. The 88 has 146hp...the 90 has 160. The 6-ports open up by exhaust pressure on the 88, and from the airpump on the 90. The Oil Metering pump is mechanical on the 88 and electrical on the 90(the mechanical ones don't ever fail or so I've heard) Thats pretty much it.

 

No difference in the end results of the ports opening they just work in different ways.

Another difference is the seat belts. The 90 might have the automatic seat belts that run on a track past your head while the 88 will have the standard style seatbelts.