National Geographic Topo utilizes a collection of USGS maps at various scales that are seamed together for the state you purchase. So the NGP maps look exactly like the USGS maps that you might find at an outdoor store (contours, colors, place names, etc.). I believe that they use 500K, 100K, and 24K(QUAD) series maps depending on zoom level. The software appears overly simplistic in nature, particularly for print functions (the mapping kiosk version is better but probably not available for purchase). All maps in NGT appear to be raster data (i.e. scanned image files). I believe each state will cost you $100 retail.
Delorme Topo USA utilizes vector data to generate it's own contours, roads, and other features for any zoom level. The software automatically turns on increasing levels of detail as you zoom in (much like GIS software can). The maps will NOT look exactly like the USGS maps from outdoor stores. However, Delorme is getting very close to emulating the USGS maps in terms of symbology, color palette, labeling, etc. The software with vector data for the entire US will cost you $100 retail.
Delorme also sells a product called Delorme TopoQuads. This software uses seamed 24K USGS scanned images that are rastered by the program just like the National Geographic software (except with no 500K or 100K maps). The TopoQuads software is standalone, OR you can take the raster data and run it within the Delorme Topo USA application. Delorme TopoQuads are sold by the state (except for a few states that are sold by the half like MT, OR, and CA) at $100 retail per state (or half).
Neither application is perfect on road and trail information. As everyone knows, USGS maps are updated fairly infrequently (maybe every 10 to 20 years?), so they are perpetually out of date on roads and trails that are built, closed, restricted, obliterated/rehabed, or renumbered/renamed. This affects the reliability of maps from National Geographic and Delorme TopoQuads. Delorme Topo USA gets road and trail information from a third party service. The updates are more frequent, but it suffers from its own reliability issues - primarily roads and trails on the maps that no longer exist. I find myself double checking new areas on Google Earth or with land management maps (USFS, BLM, State, etc) before I go into new areas because I have had too many surprises when I trusted what appeared on the software.
I personally run Delorme Topo USA 6.0 with embedded Delorme TopoQuads for Idaho. This setup is a $200 retail value, but has proven the most reliable for my needs. And Delorme Topo USA has a particularly reliable GPS interface for upload/download and real time tracking on a laptop.
I recommend the DVD versions of any of these softwares that you would select, since the data usually fits on one DVD disk. The CD versions require multiple disks which quickly get annoying.