I don't watch paint drying either, but I know it is drying all the same. If you feel the need for a scan to be run in this cyclic manner then the scan when Idle fits the bill.
I don't use either as I don't thing there is a huge requirement for either of those on-demand scans. This may well have been the case with avast 4.8 home when you couldn't schedule a scan. But now you can do that with avast 5 free, so either scheduling a Quick/Full/Custom scan to take place at a convenient time further depreciating the value of these cyclic scans.
Just my 2p worth (excuse the pun on saving power/planet/cash), each to their own.