Unless you're planning to make or take a lot of calls, the safest place for your phone would be in ziplock buried in your gear (though preferably that comes with you when you leave the bike). Sure, a crash that destroys a phone that's in a bar mount isn't hugely likely, but if one happens, that's precisely the time you'd really want to have a working phone.
So if you already have the GPS and are already familiar with it, you could risk that instead. Also the screen may be more daylight readable? And you're running down a different battery than the one you'd use for communication (bring a portably charger anyway).
Just thinking out loud, likely either works...