Hmmm. No easy answer.
MemoryMap (or similar) is the best at mapping as it uses OS maps as its base. However, the maps aren’t directly compatible with Garmin GPS machines (or others AFAIK) .
You can upload routes to the Garmin from MemoryMap and follow GPS inspired directions/breadcrumb traisl as you ride though this should be backed up with a paper map. Once you’re back from your ride, you can download your track(s) from the GPS into MemoryMap to see your route and stats.
The upside of this approach is you can use a low(er) cost GPS unit with good battery life rather than the high end, flashy stuff. Mapping is limited to what’s in the GPS and/or the bit of paper in your hand.
Another option (with crapper mapping) is a Garmin GPS with their maps. The maps are OK on the road, not very good off. These GPS’s can be higher spec with poorer battery life.
Another option (with good mapping) is to use a PDA with a GPS pod (or internal receiver) and MemoryMap. Good mapping, fragile kit with poor battery life.
The last option is the new SatMap kit, a roughty tufty PDA type device that uses its own proprietry OS mapping. Personally not used it but mapping looks dear and battery life poorer than some of the alternatives.
I’ve tried the first three options and to a GPS virgin, would suggest a cheap Garmin Etrex (~£50) in some places along with MemoryMap. MemoryMap can be ‘found’ if you look hard enough though I’m not advocating using copy software.
If you really must have a full blown map on your tiny little screen, it looks as though a PDA or SatMap is the only way to go. Don’t bother, it’s not worth the additional cost (IME).
Bob