Both are excellent. Bosio's are made in Italy and Monark is made in Germany. I have yet to see any comparison between them, but they are likely almost identical. Both are superior to the OE mercedes nozzles. You can't go wrong with either one, so buy whichever costs less and is in stock.
I use Bosio in my E300 just because they seem easier to get than the Monarks, and are the nozzles of choice in the VW TDI world which I have lots of experience with. Here's some very cool photos from the Fratelli Bosio factory in Italy:
http://forums.tdiclub.com/showthread.php?t=64198