As for the exterior as you already know, the 126 body was virtually unchanged from 84 to 90. Comparing the 90 to the 84, you might get updated instruments in the dashboard, i.e., nicer speedometer, temp gauge, etc., headlamp wipers, heated seats, and airbags. (Drivers side airbags were optional in 84 – Drivers airbag was standard in 90 with the passenger side optional on the 300 – on the 420 and 560 both airbags were standard, hence more money.) Another cosmetic difference was in 84 the trim was painted grey, but in 90 the trim was painted similarly to the rest of the car (more of a modern look). My 90 SEL was a few inches longer than my SD, so there was more leg room in the back, but your SD and SE should be the same length.
As far as driving was concerned, the SEL had about 50 horsepower more than the SD. I believe the 300 SEL has 177 hp and the SD had 125 hp. Does not sound like too much, but that is a whopping 42% increase. Therefore, you’ll feel more lift from a full stop with the SEL, better for stop and go traffic. Then you get into what is your preference, diesel vs. gas. There have been some really good threads on this board about this. Larry is an awesome source of info about diesels. (Do a search for “diesel” to read these prior posts).
In a nutshell, the diesels are dirtier (but some states don’t have emission req’ts), noisier, run hotter, but get better gas mileage and traditionally last longer if the oil is change frequently. The gas engines are much more quite and have more power and acceleration (but require the changing of plugs every year or approx 17-20,000 miles). My SEL gets between 19.6 –23.5 mpg (20.5 ave), while the SD got in the high 20s all the time it seemed. So you really need to think about how you are going to drive it and maybe consider how much diesel costs in your area. In some parts of the world, diesel is dirt cheap.
In conclusion, I think it is easier to go from a diesel to gas then the other way around. I feel so spoiled on how quite it is. It is like driving a golf cart. Much easier to have a conversation on your car phone or to the person in the pass/back seat. In the end, it is all your preference. Both are enjoyable given your usage style.
Was that the answer you were looking for?
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)