Functionally:
Both get you from A to B in the same amount of time, given legal speed limits (or even reasonable transgressions of said speed limit).
Both have basic luxuries like air conditioning, automatic transmission, CD players, etc.
Both have safety systems in place, ie air bags, restraints, etc..
Both have reasonable warranties and mean time between failures...although it costs considerably more to maintain a MB as you already know.
Aside from a slightly better ride in the Mercedes, the only difference between the two is the head turning factor. People look at you and KNOW you're making good money. In the Kia...it's assumed you're not making the kind of money you are.
Personally, I gravitate toward working trucks...but so did Sam Walton.
I have a hard time listening (reading) someone complain over a $3 increase in their gas costs per week and then happily skip into a Starbucks for a $5 cup of coffee. Expensive house...expensive car...expensive hobbies (flying, international traveling)...and you're really, seriously ticked off over $3??