I switched from NG Topo to memory-map --- I like their windows mobile app on my smartphone better than the one for NG Topo, and with memory-map (and upgrade to their base product) I can import free maps from places such as libremap, or buy them on DVD from memory-map.com

Overall the tools are similar; NG Topo used to be buggier. It was primarily a cost decision to switch, as I wanted to get maps for the entire Appalachian Trail (14 states) and didn't want to pay NG Topo prices for all of that. I haven't used memory-map extensively, but what little I have used it it's performed well, and their customer service was very good too.