Getting your Nest Cams from the Google Store isn't much cheaper than getting them from ADT. For example, you'd still need to pay for a Nest Aware subscription to get cloud storage, whereas that's included in ADT's Video and Smart Home fee.
ADT also takes care of installation, setup, and service—for a fee, however. Plus, you get 6 months to try it out.
While ADT's promotions may vary, you can typically get one or two Nest Cams for free as an extra incentive for signing a 36-month contract.
Finally, getting Nest Cams from ADT is the best way to ensure your entire ADT security system and monitoring package works harmoniously with your Nest Cams.