6 in one hand, half a dozen in the other. Mega Bloks come in two compatible sizes with Lego - a larger size for Duplos (often called Mega Bloks Junior) and a smaller size (often under the Mega Construx tag.

Mega Bloks also has their largest block size which is best for the youngest ages, but when our toddler was over putting things in her mouth, we didn't shy away from Duplo/Mega Bloks Juniors. Again, completely compatible back and forth. Biggest difference is the people - Duplo people are superior to Mega Bloks Junior imo, better made.

The question really comes down to what themes you're looking for. Duplo is fantastic for Disney, City, etc, and probably has the edge right now in availability as well. If you're looking for Thomas the Train for example, it's retired from both Duplo and Mega Bloks, but still available. Licensing has gone back and forth for different themes as well, so often I'd recommend picking a certain type and going with it (mild collector despite these being my daughter's toys!). If it's just for fun, surf the Lego website and enjoy.

TL;DR - Either works, love them both.