Why not use some of each? When I started at my work there were a couple of reef tanks with live rock. That was more then 10 years ago. Each time we've started up a new tank after that we've taken a couple of old rocks and the rest new dry rock. In Sweden dry rock is a lot cheaper

We also use as much "old water" as possible when starting new tanks.
It has worked out well for us so far. Nowadays I can't see the differens between the the old live rock and dry rocks that's been in tanks for years. So we kind of still benefit from the first live rocks we got I think.