Hi battom,
I have both right now, along with a Nuvi 660. Trying to decide which one(s) to keep. Since you asked specifically about the ONE vs 910, I'll leave out my thoughts on the Nuvi. I also won't go over the features of each, as I'm sure you are aware of the differences in the units.
The best thing about the ONE for me is its size. Truly portable. I am a road warrior and am on a plane weekly, traveling across the US to different cities. Size matters to me. The 910 could be portable, it's just not AS portable as the ONE (or Nuvi, for that matter.)
The thing I would miss the most from the 910 is the text-to-speech functionality. I'm surprised by how much I like this feature. Plus, the audio quality of the 910 with TTS is amazing.
I don't really use the mp3 functionality, that's not at all important to me. I use this device mainly in rental cars getting from the airport to the hotel to the customer's site and back.
If I had to choose today, right now, it would easily be the ONE. I do believe the 910 is the better unit, only because of all the features you get. But what's most important to ME is size and accuracy of map data. The maps are the same (or at least should be once my upgrades arrive!)
But I just can't seem to send the 910 back... that dang TTS is so awesome, and I really DO like the hands-free calling.
While not critical to me, some might consider the European maps not installed on the ONE to be a deal-breaker. I never travel to Europe.
I hope this helps you. It all boils down to what's important to YOU. They are both excellent devices. At the end of my 30 days, I may end up keeping both. The 910 as a more permanent solution in my car, and the ONE as my travel companion.
Good luck with your decision! Let me know which way you go..