Imho one evident advantage of using a tablet/phone on which you install the MRA Navigation Next app over using a dedicated GPS device is missing.
If you use MRA Navigation you do not have to export a gpx file (which version do I have to use?) in a certain format and copy thay to a device over a third party app/drive/system from Garmin/TomTom or upload them over USB to a device and hoping all information in the gpx will be used during the conversion of the gpx to a route on your device, using the maps en settings of your gps device, to stay on the route the planner intended. That's a lot of steps, one of them being the user. This is a very annoying pain in the a.. for a lot of gps device users out there.
When using MRA Navigation Next you just open the route (no gpx conversions!) on your phone/tablet, make it available offline (maps/route) and hit navigate. No conversions, so no missing data and when riding no connections needed. Easy as eating a banana.
This method also makes the MRA development team independant of gpx format definitions and limitations of gps devices. They could add everything they want. Example: add a url to a hard point in the route which gives you info about the opening times etc etc. No Limits!
If you want to use CarPlay or Android auto, there is a chance version may conflict or some new functionality is missing.
For me: tablet, absolutely. Downside: most normal tablets/phone use a USB micro or C connector, which does not fit the job. Too fragile and not watertight. Find a rugged one with pogo ins for power supply. Should you be needing Internet on the tablet for something else than MRA, use the hotspot on your phone and keep the phone safe in the pocket.