The mobile app is pretty good and easy to use. Planner provides multiple ways to look at many projects on a personal dashboard and also aggregates tasks within given projects and teams.

Everyday people are able to jump in quickly, create tasks, organize buckets, drag-and-drop items, etc. Planner is meant to be user-friendly and easy to understand. Ultimately, if cost is one of your biggest concerns, Planner can meet many of your needs at no additional licensing cost (assuming you have the right Office 365 license) whereas Project can get expensive because to even open a Project MPP file, you need to have a Project license (or a third-party app). Project Online, Project Server, and Project Professional require additional licenses (per user) and can quickly become expensive. Microsoft Planner comes free with almost all Office 365 Enterprise (including non-profit), Government (where available), and Education licenses. There are some major differences between the two tools.

Cost may be a big decision factor for you.

Microsoft project planner