Generally speaking, an aluminum bike should be lighter than a CrMo-framed bike. I have seen some low-end Al bikes that were way overbuilt, so YMMV.
Will you notice the difference? Maybe.
Will it matter? Probably not.
Unless your route has a lot of hills, or you ride really, really fast, the Belt/IGH setup will make a bigger difference in the ride than a marginal weight difference.