J. D. Power's ratings show the Lexus to be better at Initial and Midterm quality. There is no Longterm data for the Mercedes but the Lexus is outstanding.
Initial quality is 90 days and is meaningless. Midterm is 1-3 years and is important because here's where design problems show up. Longterm (3-5 years) is important because it indicates longterm reliability.
However, if you are comparing two specific cars then you need to compare their condition and maintenance history. Are they both one owner cars? Are they both from private parties or from dealers? Do they both have OEM parts like filters or some indication that they have been serviced by Jiffy Lube?
I won't buy a used car but in this case, if the condition and maintenance history were a wash, I'd take the Lexus because they are consistantly ranked the best car from the best manufacturer (Toyota).
Finally, I've driven Mercedes for the last 20 years and my next car will be a new E-Class.