I don't know about bosch losing their quality or not, but I have heard of Beru. Some people on this forum have said that the Beru wires are better than the bosch wires. I don't think there should be a problem with beru. Often if you go to two different Mercedes dealers you will get oil filters made by two different companies. As long as you trust your mechanic, and feel he is an MB enthusiast he probably won't be selling you junk. My bet is there won't be any difference in quality or longevity.
1995 E420 - retired
1986 420SEL - retired