Well, it depends on your preference. Here's mine...
I've owned a BMW 550i which I traded in for a Mercedes E550 Coupe.
Personally, I love my Mercedes more so than I loved my BMW. Sole reason I like it more is because it's more luxurious, faster, better acceleration and control with it is very... hands on I think is the best way to say it. It makes you feel more one with the road.
However, the BMW is a cheaper version with nearly equal luxury and speed and it's handling is a lot more smooth. I felt safer in my BMW as if it came down to split second maneuverability, the BMW was very top notch in its handling.
These car companies are very similar, Mercedes just likes to put more into its horsepower, acceleration and luxury while BMW focuses more on speed, handling and luxury. As a first time car, I would say go with BMW as it is considerably cheaper than Mercedes, especially maintenance.
The 3 and 5 series are more than enough for you for now. There's no need to spend 60-70k+ for a 6 or 7 series if it ends up with you not liking BMW so much. If you're considering looks of them too, don't go with a base 3/5 series, base meaning something like a 528i BMW which looks so bland. Getting a higher model such as 535i or 550i adds those extra trims, dual exhausts, etc. that just makes the BMW cars so much more appealing.