I have a '85 300D and it's a great car. However, I just purchased a 300SE and as much as I love diesels,the gas cars are smoother,quieter and quicker.
Also,the '91 is a newer,more refined car. The interior is more updated and is quieter and more comfortable. Unless you are a nostalgia buff,I'd get the '91 300TE.
84 300SD (Sold)
With proper maintainance,you'll get your monies worth and plenty of miles out of the gas version.