I am willing to buy a small amount of mega bloks and then upgrade to duplo so he can have them for independent play when baby comes. Thanks for the advice!
We have both and the kids play with them in different ways. (they are 4 and 2) mega bloks were easier for them to use independently earlier, but duplos definitely will last longer, and really for an 18 month old unless they are building a "tall tall tower" (a favorite at our house) you're helping anyway, ya know?
Duplos can be used with Legos, so have much better staying power. We found Quatros on Craigslist, Lego only made the a couple years. I think they are about the size of megablok and can be used with Duplos. If you can find the Quatros I highly recommend getting those if you think your little one is too little for Duplos. We found megablox just didn't stay together as well.