I have a Garmin 60CSx (I think) unit which I'm very happy with >>> for trail riding <<<. But I do think that your intended usage of the unit will influence your happiness.
I use mine almost exclusively in "bread-crumb" mode where I track my riding. I then upload the data onto my PC where I maintain a database of trails. I overlay the data on topomaps, GoogleEarth shots, etc. I use the data/software as a ride planning tool. I don't download topo data to my GPS. I don't consult the GPS when I ride (other that to find the truck if I get turned-around). Or sometimes I'll use the GPS to get me to a trailhead. But once I'm riding I look forward at the trail and not at the unit!
And my mount reflects that; I use the provided clip to secure the unit to my chest protector/roost guard. Statistically, I seem to survive wipe-outs better than my bike.
I don't seem to slam my chest. So far, so good. I don't think I could say the same thing if the usint were mounted on the bars.
I do use those plastic PDA screen protectors on the unit. They are an effective way to keep the original plastic/glass (?) scratch free.
I can see how one could be disappointed/unhappy with a Garmin Nuvi type of road-unit in the dirt. My understanding of the units is that they're not set up for trail use/bread-crumbing. But I have no first hand experience with one (other than on a road trip).
One "fun-feature" of my 60CSx is the barometric altimeter. Yes, I realize that it only measures atmospheric differences and is literally effected by the weather. But, overlooking that basic limitation, having altitude info is fun. There's a site (gpsvisulizer (?) or something like that), which - among other filters - will colour your track by changes in elevation. The coloured data can then be overlayed on GoogleEarth.
Have fun with whatever you buy.
Dale