I've got an Ambit 2S, I think the battery is the same in the 2R - you can get about 8 hours out of the battery on high GPS sensitivity, to run an ultra you need to set it to lower updates - which means your distance measurements are going to be less than the real distance. Not a huge problem, more of a slight annoyance. (To give you an idea of the difference, I did a 102km ultra, and the GPS data shows 99km).
I used to have a Garmin FR210 - nice watch, although not fully waterproof, but the HR strap used to rub me raw: at the end of the Madrid marathon my chest was bleeding from the area at the bottom of the strap. The Suunto strap is much more comfortable. So just for that alone I'd say go with Suunto.
Other points - the Ambit 2R (and 2S) can estimate treadmill distance using the internal accelerometers, no idea if the Garmin can. It also looks like a watch (unlike the Garmin 910XT, for example), which may or may not be important to you. The Movescount site isn't bad, although I usually end up uploading the runs to Endomondo. (Downside: Suunto doesn't support linux, and there's no direct import into Endomondo (unlike Garmin)).
The original Fenix has a terrible reputation, my local shop has said they will never, ever stock Garmin again after the trouble it caused them. No idea if that is true for the Fenix 2, though.