You are correct - they are very much the same car. You mentioned the front sheet metal. One thing to keep in mind is that the 1990 740 has front end sheet metal that is unique to that year only, possibly making future repairs more difficult. The 1991-up 740 shared the same sheet metal as the 940. I guess this is only an issue if you are prone to running into things.

I believe there are some other differences in the transmissions (torque converters) in that some were lock-up types. But I can't tell you with any confidence what years and turbo vs non-turbo, etc., had which torque converter. I don't think it's a big issue which one you get as it seems either one was a good tranny.