I've always used a mounted 5" Garmin. I've never used my iPhone for traveling, not even in my car. The Garmin automatically scales it's display better and is much easier to read, plus lane assist is a huge plus when driving through cities. If you have the Garmin mounted right, you don't have to take your eyes off the road.
I've also never used a MC dedicated Garmin. The one in the picture is an old Garmin Nuvi 1450 (Nuvi's much cheaper than a Zumo). Fortunately where I live, it doesn't rain that much so if it starts raining, I'll just pull over and stick it in my tank bag.
Now, having said the above, if I was doing touring all over the US like I used to do, or lived in a rainy area, I'd get a Zumo so as not to have to worry about water proofing.
Edit: The other huge advantage to a Garmin is the ability to import custom routes that you make on your PC using mapping software.
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