LEGO has better quality plastic and quality control in general. MB tend to get stress marks in the plastic, often have a few missing/deformed, and can get stuck together for pretty much good. But I will say MB CS isn't terrible and they were far better than Kre-O blocks.
LEGO tends to give a shit about everything. I had a missing head in a blindbag. They sent me a new one and a giftcard for the problem. Almost never a missing pieces, though. The bricks connect and unconnect very well. Never any stress marks on the plastic.
