I don't know much about Mapsend Topo, but I do have Mapsend DR which I use in my Magellan Meridian Colour and it is marvelous. I've used it all over western Canda and the US and it registers even dirt roads and small pothole lakes etc. with very good accuracy. I wouldn't get the Topo unless you intend to do a lot of offroading, hunting and general bushwacking. The base map supplied with the unit is reasonably accurate if zoomed in, but pretty much useless for day to day use.
Matt