I've used a Satmap Active 10 for a number of years. It has pros and cons as you would expect. I've recently reverted to using a Garmin however, for a couple of reasons. I found the waterproofing of the Active 10 to be inadequate. They have in effect acknowledged this by recently marketing a waterproof case for it, although the design precludes using it with the bike mount. No doubt if you're a handy chap you could make a home made workaround, but it demonstrates one of the primary characteristics of the Satmap which is that IMHO it's designed primarily for walkers rather than cyclists. The Satmap is for example quite good at the things geocachers like. I could go on at length on this subject but I won't bore everyone by doing so, but it's the little details that make the difference in the long run and you find these things out only when you live with a device.
The recent generation of Garmins have caught up with the Satmap in a material aspect, that of now being able to accommodate OS mapping. I have recently acquired a Garmin Map62 with OS mapping and I find it pretty good. Certainly the build quality seems better than the Satmap. I had a Map 60, the previous model for many years and it gave me good service. I haven't used an Edge 800, which also can have OS mapping, but I suspect the screen is on the small side, as smurphboy said. The Satmap has the advantage in this regard, even against the Map 62 (although less so there) but I have a problem with the screen legibility on the Satmap in brightish light. To be fair, all the OS mapping handhelds suffer to some degree from this (although I have no experience of the Memory Map one, so have to reserve judgement there.) Both types have an adjustable backlight but the Garmin is superior in strong light.
The buttons on my Garmin are easier to use, especially with winter gloves; those on my Satmap are indistinct and unresponsive; I had one break on an earlier unit. I believe this varies from example to example though.
With regard to battery life, both the Satmap and (some of) the Garmins run on AA cells. The Edge has to be recharged however, after about 15 hours. Some people can get around this using portable power packs. It depends on how you use the thing. If you have time to stop and recharge from an external pack, great. Doing it on the move may present weatherproofing issues; you might be content to bodge around this; some people do.
The Satmap is good for making up routes on the fly. You just pan around the map plonking waypoints along the way and it draws straight lines between them. Rudimentary but adequate. On the Garmin you can do this only one step at a time. But the Garmin will allow you to pan to a distant point and mark it as a "Go to" and it will then navigate you, satnav fashion, along the road to that point. The Satmap cannot do this. You may not however much like the route the Garmin plots however, as there is a limit to its intelligence. With a bit of practice however, it will get you out of trouble. You can compose routes on your PC for both devices. The Satmap advertises a "Route Planner" I tried this when it was first released and didn't think much of it. As I've always had Memory Map on my desktop I stuck with that and haven't tried the Satmap thing again; it may have improved. There's a lot of informative chat about the Satmap at:
Pocket GPS World - SatNavs | GPS | Speed Cameras There's also a Memory Map Adventurer forum there too.
For cycling, the bike mount can be important. The Satmap one is an Abus/Rixon & Kaul klickfix style bike lock mount; it's ugly and rather cumbersome but it never failed me or broke. I did however, make my own version using a USE Exposure lamp mount because the Abus one took up too much handlebar space. In comparison, the Garmin mounts I've had have been a bit flimsy and not wholly reliable, but forewarned is forearmed and so long as you use a safety tether, it's OK. (I speak only of the Map 60/62; the Edge 800 is a much lighter unit and shouldn't strain its mount so much. Be aware though that it requires a 90 degree turn to attach/detach, so fitting it close against something else might be an issue.) I've not used a Memory Mount but the one I saw at the Bike Show had a suspect looking mount. When mounted on a handlebar, these things take a hell of a hammering from road vibration and a well designed and robust mount is essential.
I haven't studied the matter of continental maps. None of these things are much good without decent maps and I can see how availability of same might swing it either way despite the relative merits of the respective units.
I could blather on at even greater length on this but as a broad summary, I'd say that I used to recommend the Satmap until the Garmins got OS mapping, but now I'm back to Garmin. If I was starting from scratch, I'd research the mapping availability first and then think very carefully about exactly how I intended to use the unit and research extensively.
I've had occasion to use the service and repair departments of both Satmap and Garmin. Both have given pretty good service in fact and the Satmap people are here in England and easy to speak to. On two occasions however, Garmin have generously supplied brand new units without charge when I had a problem. (One of those was when the thing flew out of the bike mount and bounced down the road. It still worked perfectly but was a bit scarred. I send it back to them complaining that the bike mount needed a safety lanyard and they sent me a brand new one gratis, which was pretty decent really, I thought.)
One final point. You can download the respective Satmap and Garmin user manuals to try and get a "feel" for them. The Satmap is much improved of late in this regard but the Garmins remain crap; they are legendary for useless manuals, which tell you very little about what the kit can do or how to do it!