I belive Notify17's UI is friendlier for users, less cluttered. Clearly Notify17 has less features overall (for the moment), because it focuses on few core concepts, which are simple notifications ( https://notify17.net/docs/api-endpoints/#raw-notifications ) and notification templates ( https://notify17.net/docs/templates/ ).
The two products have two distinct philosophies. At the time I evaluated pushover, I thought it was too enterprisey, not suitable for simple daily usage. Therefore, I tried to create a friendlier service :)
The tradeoff, for the moment, is probably less features overall (you can see a list of features here: https://notify17.net/docs/features/ ). The benefit is that you have a UI that lets you manage your content (in my opinion clearly) way far more easily than with pushover.