It's a personal preference thing. It matters not one bit which one you want to use as long as it works for you.

From what I've read on other forums, phones might be a little better for use in cities with tall buildings. The idea being that the cell tower connectivity gives them a quicker recovery from signal interruption. Some say that in areas far from cell tower reception, that dedicated GPS cyclometers fair better.

So if you are like most and not hampered by very tall buildings or out in the middle of nowhere, then it boils down again to just personal preference.