We are big duplo fans in this house so I would say run with the duplo. It allows them to do a little bit of building but has more pieces that they can just play with without having to do too much first. My DD is 6 and has some awesome lego which she loves, but she will still get the duplo out & play with it. Big kids love it & play with it if its there too. It's so chunky & satisfying in a tactile way that little lego isn't. They've got some lovely princess & castle stuff available now which adds another dimension for imaginary play.