Joe,
In my experience of several years comparison on many long trips that the Garmin is superior to cell apps.
The route choices sometimes vary quite a bit so, my bro & I have ran tests where he followed his cell & I my Garmin.
I usually arrive first
(our rule is no more than 10 over) and my routs are usually more scenic. There is more but, I won't bore you with it.
I have used both in my vehicle business travels and prefer the Garmin.
Both GPS and telephone are important. Try both at the same time and you shall do each half-ass!

JMHO, but I could be correct!