I don't think it is necessarily and either/or situation. There are advantages and disadvantages to both.
I've used both and prefer the stand alone GPS. Larger screen and it seems more precise when driving. Someone mentioned a delay in the smartphone navigation and I've experienced the same thing. Really sucks in some larger cities where you REALLY don't want to get off at the wrong exit to re-route.
I also like the larger screen of the GPS compared to my smartphone. Sure, I could get a tablet, but I don't want or need a tablet so it doesn't make sense to buy one just to get a larger screen to use as a GPS. I also don't like the idea of having my phone in one of those cradles on the dash in the event of a crash. The odds are much greater that the cellphone will go flying and get damaged/destroyed than if it is in its cradle on my belt.
I'm also curious (if anyone knows), when you use your smartphone in memory mode where the route is stored, how does it know where you are for navigation purposes or is it just a map at that point? It would seem like it would have to use the location services on the phone, which it turn use data and for which you have to have cell service, but I don't know.