I'd choose my phone over a dedicated Sat Nav every time - and for one very simple reason. Dedicated Sat Navs are all widescreen. Why on earth would I give a damn about all the things to either side of me when I'm driving? I want to see as far ahead as possible so I put my phone in portrait. I've always found it baffling that everyone produces Sat Navs in widescreen.
For the record, I've used TomTom, CoPilot and NDrive quite a bit. They all seem to do the job. Don't use TomTom any more because of the awful licensing (as mentioned above). Not a lot to choose between NDrive and CoPilot except the contacts integration is better on CoPilot - although I think my NDrive is now one version out of date so that might have changed. The NDrive roundabout instructions are a little weird but on a recent holiday driving quite a bit around Paisley, East Kilbride and Newton Mearns (none of which I knew but appear to have been designed by someone with a roundabout fetish) it did a great job.