I like the E34 shape very much. I miss my M5 quite a bit. The S38 motor was a jewel of a motor and just sounded awesome. 525 is dreadfully underpowered though. The car weighs nearly 4,000lbs.
I had considered an E300/300E - the one co-built by Porsche...but in the end, the M5 was more to my liking because it was a stick shift. I think the performance between the MB and M5 are very close.
I have never had the pleasure of driving the MB, but I can tell you the BMW is fantastic. You have to beware of the 540i's V8 and the Nikasil motors that were known for failing. Google it and there should be discussions on it.
For these older cars, you get what you pay for. Pay for the best car you can afford and make sure the paperwork is in order.
My old baby:
http://www.bmwmregistry.com/detail.php?id=1177