Apollo is roughly an order of magnitude cheaper than ZoomInfo at typical seat counts. The contact data is good enough for most outbound use cases, and you get the engagement layer (dialer, sequencer) bundled in. For startup and SMB outbound teams, this is the obvious pick.
ZoomInfo's edge is depth — buyer-committee mapping, intent data, technographics, deeper coverage of private companies and specific industries. For enterprise sales orgs whose territory plans depend on knowing exactly who reports to whom inside a 10K-person target account, ZoomInfo still wins. Cost is multiples of Apollo and the contract is annual.