If you are not going to use it for navigating, only plotting onto a map Google Earth after you have been hill walking then any of the smaller Edge GPSs or the Forerunner are fine. I have put my Edge 500 in my rucksack pocket and taken the data off it later.
I also don’t see the problem with the running watches as a bike data logger, unless you want cadence or power output information. They can be strapped onto the bars pretty easilly and if you are going to do some running with it then it is the best option. Holding an Edge is possible while running but a bit of a pain.
If you really want to navigate in the hills with it, then, like Druidh says, i would (and do (gpsmap 62st)) go for a handheld with a handlebar mount for the bike. I do concede that the handhelds can be a bit bulky and you would need two bike mounts, which Garmin sell for silly prices for what they are.
Different strokes for different folks.