Unless you can mount your phone where it's display is in view, is a sat nav app really usable. Whilst you are listening to directions, I find it is very useful as you approach a junction to be able to glance at the screen to appreciate which way the thing is trying to send you so you appreciate how you are going to negotiate the road ahead. On that basis I would always prefer the dedicated device.
Systems such as TomTom are well developed and with regular updates available, even allow you to keep the maps, speed cameras, etc, up to date for no too great a cost. But as a dedicated device, if you want portability, especially for walking around, then perhaps the phone app is the better option?