I'd say get the Quatro this year and the Duplo next, because they are actually compatible with each other, so having both makes sense. My experience with Duplo is that it's really more for about 3 years on up (DH still likes to build with them!). They are a little difficult for tiny hands to manipulate.
The Pop-Onz are pretty good, but they don't stay together as well as Legos, so if you bump your creation the whole thing will fall apart.