I've got the '89 benz 300E and use to ride in my friend's '92 3-series.
looks/image:
benz - classy from bumper to bumper / for the old (real) rich
bmw - little less class, little more sport / for the new rich
performance:
benz - good enough, nothing special, handles very well in dry roads... can't say the same in snow
bmw - harder shocks, bit more power:weight ratio (I think), fit for the racer
ride:
benz - pure comfort
bmw - comfortable, but not as good as 300E
upgrading/repairs:
benz - lets not go there... expensive as HELL!!! $15000 for engine upgrade!!! / very well built though, it's only been at the shop once because air conditioning broke.
bmw - still relatively expensive, but much better than benz / still very sturdy car, but I don't think it'll beat the durability of a benz.
to sum it up, it really depends on who you are and how you like to drive...
I'm 21 too and I very much prefer my benz over the bimmer. I often have important meetings where I need to wear a suit so the benz is more fitted for this lifestyle... there are other days where I feel like RACING the day away and I could do that no problem as well. Problem is, if you don't know how to drive the benz, you'll think the acceleration sucks (because it starts in second gear). They do this cuz it's a luxury car and don't want to glue you to the back of your seat. If you get it to 1st gear, you'll be blasting off like CRAZY!
Everything is very durable and will last quite a while. my car just reached 12 years old and top speed could still reach 135mph. not bad for something that's bone stock.
can't say much about the bimmer cuz I've never actually driven it, I know it's a NICE car though!
btw, the Benz 300E is an in-line 6 cylinder. don't think it's a V6