Having used a previous phone (Windows smart phone running TomTom Navigator) for bike sat nav, though only as emergency back-up to my then Garmin StreetPilot. I would say that a dedicated navigation device is a better, though maybe more costly, option.
A phone is that first and a sat nav second. The software available isn't as capable, in your case probably
CoPilot is probably the best bet. The buttons on screen will probably be too small for gloved operation even if your phone's screen will work with gloves (my S2 is sort of OK with thin summer gloves). You will need to arrange waterproofing and power. Any incoming calls will probably disrupt navigation...
It's a matter of compromises. The guy I usually tour with uses a Nokia phone as a sat nav, usually does it's own thing route-wise compared to the routes I've planned for my Garmin (he has to manually recreate them...). It insists on visiting every plotted point so if he's put one off the road, or we decide to adjust the route, it keeps turning you back until you manually cancel the missed point/s. He has to play a looped silent MP3 track on his media player to keep the Bluetooth channel open so he can hear navigation instructions etc... OK, I accept that some of these are down to his cobbled together set-up. At the end of the day I would prefer to have a system that pretty well works out of the box and doesn't add complications whilst on tour.
I can understand the intrigue of using just one device and wish you good luck if you go that route (sic) but I doubt you'll be getting the best nav experience.
I do have NavFree UK and France on my S2, for dire emergency back-up whilst on tour or occasional cage use in the UK.