Go with Duplos. They are far superior to Mega Bloks. We have a huge bin of duplos and my kids still play with them all the time. They are incredibly well made. The problem with Megabloks is they just don't stick together as well. Trying to build something and having it continually fall apart is extremely frustrating. We have owned Megabloks of all sizes and I have come to realize they just aren't worth it. The little tiny ones are the worst. DS got a couple of the Halo sets and they made me want to pull my hair out and take up drinking.
DS 10, 5th grade
DD 8, 3rd grade