Tablet
Way more options for apps
Way more useful.
...Proprietary device limits you.
I see a benefit to both. While mobile devices (tablet/smartphone) offers a lot of customization and app selection, one thing that really bothers me is configuration complexity, and reliability.
There is a trade between Flexibility and Reliability. You don't really get the best of both in a quality device, it either does exactly what it is meant to do without fail, or it lets you tinker with it and works fairly well as long as you stay ahead of it.
A dedicated device usually has extensive mapping storage pre-installed. Even if you can update it, you generally cannot screw with it's content or coverage. All of the USA for example. While you can load large maps to a mobile device, will
you (the operator) do so reliably? Will it work when the device looses network connection? Will it be recoverable if the device software crashes and requires a reset? Can you get it back on the fly if data becomes corrupted?
My experience with mapping apps on mobile devices is while they work well most of the time, they are prone to occasional glitches and user error. Supposedly downloaded maps suddenly are not available offline, features don't work as expected. Accessing mapping files is clunky and haphazard. While this stuff can be sorted, in a travel situation you neither have the time or patience to figure it out at that moment you need it.
Also consider risk. You use a multi-purpose device for multiple things, thus it spends time on your person or transferred from your vehicle to your lodging arrangement. It is more likely to be damaged, lost, or stolen while being transported more frequently.
I use a variety of devices and software for different situations. One thing that is consistent though is a Garmin dedicated road nav GPS is in my vehicle when I'm traveling, regardless of other devices in use. Should all else fail that device will work and has mapping for any location I'll be in. Even if it's not in use, it is a reliable backup with a low likelihood of malfunctioning simply because it only does one thing. It has a simple and robust operating system software that is uninfluenced by outside parties.
I could go into long lengthy detail, but I typically have access to both a tablet and a dedicated GPS when traveling, and use whichever one is more suitable for the situation.