The main difference is that a priority onboarding by Boom ensures you will be able to start within three days and there is no risk of the 90 day expiry of your onboarding service. Everything else is exactly the same, from pricing to support.
In almost every way, a priority onboarding and an onboarding provided by HubSpot are exactly the same regarding pricing, call cadence, duration, and support type. The key difference between them is that with a priority onboarding provided by Boom, you are guaranteed to be able to start within three days and there is no risk of the 90 day expiry of your onboarding service as with a HubSpot provided onboarding.