I have tried a go510, nuvi360, and ended up with the go910.
Dropped the 510 because of goofy routing, map errors and my wife wanted the text to speech.
Loved the nuvi form factor, and the trip statistics functions (avg speed, time spent moving, time spent sitting, etc) but dropped it because of goofy routing, map errors, and it only allows 1 via point. It does not allow you to make up itineraries like the TomTom unit does.
The 910 is the best compromise for me right now.
As you can see you will find goofy routing in both units, and both map companies have errors that will drive you crazy. The TomTom intinerary function minimizes this by allowing you to force it along routes you know to be better or fits your driving better. For example, sometimes the TomTom will route you through a bunch of residential streets because it is "Faster".
But it makes you take 7 turns where taking 1 other road may take 3 minutes longer and only take 2 turns. My wife likes to keep things simple even if it means taking a few minutes longer.
Neither unit makes sense of HOV lanes, the Garmin kept wanting me to exit immediately and get back on the main highway, the TomTom has the wrong identification, and wants me to take too early of an exit. I save 3-4 minutes by staying on the HOV for another two exits and then going home.
If they made a 910 XL, (new slim form factor first in the ONE series) I would have bought that.
I bought each unit from amazon except for the 910 which I got from Dell because of a $100 off coupon. But yours from a place that has a good return policy and try it out.