I've used both the Garmin 705 and the Satmap.

The Satmap is good, but I've found the batteries aren't great and it's pretty huge. It takes three AA's but that huge screen eats up power and I found it wouldn't last more than about 6hours.

I also find the controls on the Satmap a bit unintuitive compared to Garmin.

I also have used the Garmin 705 which is very similar to the 605. I used this on a Killer loop last week and it was very good — and the battery lasted the 8hours we were out with juice to spare (although you'd need a charger on a C2C).
The problem with the Edge series of GPS units from Garmin is that they will only accept 100 waypoints when you create a route. The etrex series will take 1000 which is far more useful for off road routing. What this means is that you have to create the route as a track. For an explanation of the difference, see here: http://gpstracklog.typepad.com/gps_tracklog/2005/12/tracks_and_rout.html

I created a track in Memory Map (although you can do the same in any mapping software) as a GPX file and uploaded it to the 705, together with the relevant map tiles from Garmin's own Topo GB software. This mapping is nowhere near as good as the Sat Map OS mapping, but it's good enough.
Out on the trail it's easy to see where you are in relation to the track you've created and it's obvious when you have gone off trail. Obviously I also carried a paper OS map — a proven fail safe — but didn't really need it. It's just nice to have.

The biggest thing with any gadget is to get used to it prior to the event. Get it set up and take it out on an easy route before the big trip so you know what's what.