one other thought about where to carry a cell phone. If you should fall or have an accident, are you going to be able to get at your bike unless you are entangled in it? are you going to be functional enough to figure out how to unzip or open the seat bag, or wherever you have it?

Of course if you fall hard or roll it will probably fall out of your pocket as well- but perhaps in a pocket is marginally more accessible in general?

just thinking out loud.