The polini cases don't have the oiling holes for the bearings bored and the reed valve isn't as good in my opinion.
I built the polini, it went together ok but if it was my choice i would have built malossi. I think the polini is matched out of the box to the polini cylinders, so that might be one reason to go polini (if you really want to run something like the W port) but it takes a fair bit of cleanup.