E90 is less fussy because its been around longer, there's more E90s on the road than A Class. More workshops, and a lot more of the E90 issues are known, which makes it easier to maintain. Plus 2010 is the LCI model, which means whatever major issues has been fixed by BMW.
Buy it, get all the necessary wear and tear parts replaced. And you're done. The 320i engine has issues, so if you want the 3 series get the diesel which is great, or the 6 cylinders.
Both are very different cars. One is the mainstay of the brand, the other a car designed to fit a specific (low) price bracket with a premium badge.
Maybe you should go see the car in person, sit in it and see which you prefer in terms of cabin comfort, since you don't care bout the drive. The BMW is spartan, whereas Mercedes are more premium. See what YOU like.