Hi Maja Møller Christensen,
It is possible to implement custom pages that can be embedded in forms such as canvas applications. I did the following test:
1.Go to Power Apps and select your Canvas App, then click on Edit->Edit in preview.
2.Select the custom page and click on Edit.
3.Find the App ID of this page and copy it.
4.Select the Table, which includes the form where the canvas app needs to be shown. Select Forms and choose the Main Form.
5.In the form designer, select the section on the form where the embedded canvas app needs to appear.
Either select + Component from the top menu, or select the component icon from the left hand menu. This opens the components pane.
6.Paste the copied App ID into the Static value of App ID and save it.
7.Save and publish.
About which would be better, this is based on our business and application scenarios Canvas Apps are designed to build a flexible user interface that connects to various data sources easily, whereas model driven apps are designed with an out of the box business model that you can easily extend and integrate.
