The 740/940 series is one that seemed to actually improve in later years - somewhat of a rarity in the auto world. I've owned both a 1990 sedan (sold) and a 1992 wagon (wrecked last week) and would buy another 90-up wagon in a heartbeat if a nice one surfaced. U.S. market 90-up non-turbos have the Regina fuel system which some people don't care for, but I like it. They say the O2 sensors are more expensive, which they are, but the price differenc isn't a show stopper and they RARELY go bad on a Regina. Plus I don't ever recall hearing of a failed Regina (Bendix) ECU, and there is no mass air meter to go bad at the tune of $250-350. If you buy an 89-up, regardless of the fuel system, you'll get the OBD system which can "help" locate system problems, albeit you need to kinda read between the lines when deciphering the error codes - it's still a help. The 90 will likely have a drivers air bag where the 86 won't - you'll have to decide for yourself if that's good or bad.

At the end of the day, there probably isn't a whole heck of a lot of difference between the two. Purchasing the one in the best condition is more important than the year. Don't be afraid to consider the 940's in your search - they're just a refined 740 if you get one with the B230 engine.