Phone is great but a lot of the time I don’t have any mobile data reception (darkest Westcountry) and therefore maps is useless. I also like having a bigger screen.
Seeing as how I spend a ridiculous amount of time in and out of Devon, Cornwall, and south Wiltshire, where even a phone signal, let alone data, is hopelessly inadequate, I use apps on my phone which have all the maps installed on the phone. That way there’s no bother about trying to download a new route and getting nowhere. You get map and app updates and live traffic because it’s all on the phone. All you need is a £1.25 screen mount and a cable to keep it charged, either from a USB or 12v adaptor and you’re good to go.
I’d much rather use my phone than a standalone, there’s much less chance of my leaving it in a car, which one of my team has done…
I use Navmii and Here, currently Navmii, it seems to function rather better, and the maps are clearer. I used to use CoPilot, but reviews of the latest updates to the paid for version are less than encouraging. Navmii will give you alternative route updates on the fly, which is nifty.