Should you go for a bike specific GPS, look into the planning software.
I read that the Tomtom uses "Tyre" a Google maps based program. I regularly use google maps as I find it the easiest to use however it does have a drawback.
About this time last year I planned out all my routes for our Alps trip. I spent hours on it over a few weeks. As winter set in I noticed a few of my routes had changed. As Google maps is "Live" as roads/ passes close due to snow/ landslides etc the maps change automatically. These were saved in my account too. It effectively undone most of the work I'd put in. According to Ride magazine, the Tyre software used by Tomtom will do the same.
Garmin has its own software which is no where as easy to use as Google but will not alter your planned route.
Just a thought.