Difficult to answer which is the best gps for sub saharan use as there's so many variables but I can tell you what I've done. On my last trip to the area I went with an old Garmin GPS2+ which will do lat/lon and not much else apart from waypoint to waypoint navigation. I also took loads of paper maps - IGN 1: 1,000,000 mainly, as well as tourist type ones. I only got lost once and 2 mins of cross referencing lat/lon from the gps to the map identified the error.
As soon as I got back planning for the next trip started and a gps upgrade was part of it but as you've probably found gps mapping of SSA is thin on the ground. Having looked at all of the Garmins I concluded there were no suitable maps available (although this was 2 yrs ago and things seem to have improved slightly since then). There are however plenty of paper maps and stuff like the Russian military maps available in electronic form so it became a question of what hardware is needed to run these.
I ended up buying a Mio 550 pda and running Ozi explorer and another programme, GPS Tuner, on it. The 550 has a gps receiver built in and will hand the position data on to whatever mapping software is running.
The Russian military series are available on the web in Ozi format and GPS Tuner will run maps as jpeg image files that come from scanned paper maps (or even Google Earth) after calibration. It will also run normal TomTom sat nav for Euro use so that gets used until Morocco when a change of sd card gives me the Ozi/GPST maps.
PDAs are pretty fragile though, particularly given the sort of hammering everything gets on a long bike trip and that has been a consideration. The old GPS2+ was just fixed to the front rack with a bungie strap and survived rain, sandstorms, falls etc without major problems. The pda has had to be protected inside an Otterbox which has then had to be modified for power cable access and screen washout in bright sunlight and a downside is that this makes for a fairly large lump on the handlebars. So far (around a year) it's survived and works well. The only problem is with my ageing eyes being able to focus on the road and the screen with the same pair of glasses!