Duplo v Lego which is best for 18 month old?

[Pocket1]

Trying to get ideas for DD for Christmas and I can't tell the difference. Are they interchangeable? Is one better than the other?
TIA

Definitely duplo. Also duplo over megablocks at that age too. We chose megablocks for dd1 which was a mistake as once she was 3 she was pretty bored of it and ready for proper lego - she did play with lego with older cousins at grandparents. As she was 3 it would have been pointless to buy duplo though if we had bought it earlier she would still have played with it - again plays with it still at the grandparents with dd2.

Megablocks not megapixels ffs!