I have both the Memory-Map Adventurer 2800 and the Garmin Etrex Vista Cx.
Swings and Roundabouts; but on balance, the Garmin is better and more useful, although the Garmin MapSource maps are not as easy to read as the OS Landranger ones on the Memory-Map, especially if off-road. (Cycle paths, bridleways, byways etc.)
The procedures to up/down-load routes/tracks/maps to/from the Adventurer are so complex that I've saved the pdf instruction sheet onto my PC, so I can refer to it each time I need to do these actions. It's very odd that Memory-Map on the PC doesn't recognise the Adventurer as a GPS!