Either car will be fine, but it will depend entirely on the previous owners' care and feeding. A papmered 100,000 mile mazda will be nicer than a 20,000 mile benz that's been beat to crap. I'd check with the local BMW club for their classifieds (www.bmwcca.org for national site). If they are a member of the club, chances are they take pretty good care of the car. Not sure if the benz club is as strong.

I own 2 BMW's, one is a 94 325i with 100,000 miles that I have had since 18K. Of course this is a sample size of one, but it has had a few minor things, the biggest being the air conditioning compressor which was replaced last year. Overall, it does the job just as well as new, esp. if I repaired a couple of minor electrical gremlins. I think the value in a used BMW or Mercedes compared to a newer car is pretty good, but you have to expect higher maintenence costs. BMW and Mercedes don't really hold up any better than any other car, it's all in how you treat them and how lucky you are. A BMW will ride nice at 100K miles, but you have to replace the tires and shocks just like you would in a ford. A nice mid-90's 5 or 7 series is a pretty good value right now if you appreciate a luxury car and are willing to make the trade-offs that come with it in maintence costs. Not really sporty though unless you are talking M5, need to check out a used Porsche for that (my 3rd car).