The MB-Branded Bosch plugs are Platinum, while the MB-Branded NGK plugs are Iridium.
Iridium plugs are generally more expensive, and have a longer service life (according to NGK) of up to 100,000Km (around 62,000 Miles).
However, given that MB stipulate plugs change every four years anyway, in reality it does not matter much if you go for the Bosch or NGK plugs.
I bought a set of MB-Branded NGK plugs from BRABUSK8, and they were very cheap, so thanks Acid!