Scrolly vs Brick is the question people ask most before buying either one, and three different people have already answered it at length. This page points you to them rather than summarising them badly.
The mechanic is the same on both devices. You pick the apps you want blocked, tap your phone against the device to switch blocking on, and tap it again to switch it off. Neither is a lock you cannot open. The differences are in price, in what the app does around the block, and in whether the device is built to be carried or left in one place.
Drew Hines runs a straight which-should-you-buy verdict, including a section on where Scrolly falls short. Lynch On Life covers the same ground as part of a wider review, with a side-by-side on mechanics and price. Both are on the main reviews page, with a link to the original in each case.
For a feature-by-feature breakdown - price, schedules, custom modes, analytics, emergency unlocks, and the motivation features each app builds around the block - Scrolly has published its own comparison. It is written by Scrolly, so read it as the maker's case rather than an independent test, and weigh it against the video reviews above.
Reviewers of both products keep landing on the same point: the result depends less on which device you buy than on where you keep it. A blocker within arm's reach is easy to undo. The people who report the biggest change are the ones who put the device somewhere inconvenient on purpose.