Whitening carried out at the dental practice ( Zoom is one example) usually works on the visible surfaces. Tray systems used at home usually work on the whole arch of teeth.
Complete Zoom! treatment packs provide an additional 2-3 night tray system to to top up the work done in the chair.

The end result of both should be similar if the procedures are correctly followed but the route to acheive it boils down to personal preference.

As a clinician I might prefer one to another if the patient has issues with sensitivity or want to match to the shade of existing porcelain crowns / veneers.