Placeit's case is breadth. It has the largest browser mockup catalogue in this comparison, heavy on lifestyle scenes with real models, and it is aimed squarely at people selling merch rather than at general design. If the scene you need exists anywhere, it probably exists here.
Canva's case is that mockups cost nothing. They arrive inside a design suite most sellers already have open, the library grew considerably when Canva absorbed Smartmockups, and there is no separate subscription to justify.
What makes this comparison unusual is that the strongest argument against the paid option is not about the product. Placeit's Trustpilot score is 2, and its one-star reviews are overwhelmingly about cancellation and unexpected charges rather than about the mockups, which reviewers largely do not complain about. One two-star reviewer sums up the shape of it: the site was generally good for years, and ending the subscription took weeks. That belongs in this decision, because a subscription you cannot leave is a different product from one you can.