uuuuuuhhhh..... Discones are omni-directional and unity gain. The Scantenna is spec.d at 4.9 dB 'gain' for VHF-lo only (and they don't specify dBd or dBi). And only Stark Electronics posts that "4.9 gain" detail, no one else does that I've come across.
The Scantenna is lower cost, but you'll have to adapt from F to BNC to connect at the scanner end of the supplied 50' cable. If the balun goes, which apparently they do pretty easily, that will have to be replaced periodically.
Size comes into play also. Some discones have a whip on 'top,' others don't. The Scantenna is overall pretty 'tall.' The other potential consideration is a fiberglass omni which wouldn't have elements 'sticking out,' just its height to be taken into account.