Pro 420 SEL: Rear seat legroom, slightly better ride due to longer wheelbase, greater engine displacement makes holding speed on long hills easier.
Pro 300 SE: Feels a little more portable around town, slightly easier to park, better gas mileage, much simpler engine architecture.
VERDICT: If you live in the mountains, and / or cart around adults in the back seat on a regular basis, 420 SEL, but you pay a little more for your thrills. Otherwise 300 SE.
I'm assuming the passenger-side airbag was around in '89. If not, you may want to stick with '90-'91's.