Scope. Plausible measures web traffic. That is the whole product: visitors, sources, pages, goals, a dashboard somebody can read without training. Matomo aims at feature parity with Google Analytics (heatmaps, session recordings, app measurement, custom reports, a tag manager) and is a much larger thing to learn and to run.
Cookies. Plausible stores nothing on the visitor's device, full stop. Matomo can be configured that way and is not by default. If your reason for leaving Google Analytics is the consent banner, that difference is the whole decision: one of these removes the question and the other makes it a configuration you have to get right and keep right.
What running it costs you. Both are open source and both can be self-hosted. They are not equally easy to operate. Matomo is substantially the heavier, which is the price of the feature list. Plausible's cloud is the simpler path and it is a subscription; Matomo self-hosted is free of licence cost and paid for in server, upgrades and attention.
Money. Plausible has no free tier: you pay from the first day, on a cloud plan. Matomo is free if you host it yourself and sold as a cloud plan if you do not. "Free" therefore means two entirely different things in the same sentence, and the honest version is that Matomo's free option has an operator's time in it.
Apps. Matomo measures mobile apps. Plausible does not, and does not claim to. If software people install is part of what you need to understand, this ends the comparison on its own.