Geez, what you miss in a day or two! I'll respond to a few of the issues that have come up that I'm a bit familiar with:
Nuvi: Yes, it's POSSIBLE for the Nuvi to run Garmin's topo maps, but the unit is not optimized for this and the result looks crappy. I would at least tell you to try it before you buy it if that feature is important. However, even with topo capability in theory, it's not an ideal choice for offroad: it has no track-back, as has been mentioned, but also only one waypoint allowed per route--yes, that's no typo, only ONE waypoint. It is a great car unit for street nav--I believe the best one currently available--but that is really all it should be used for.
Lowrance: They make quality gear (and at a very good price). The Baja or the iWay 500c (or better, the 600, which I think is due out this month) are worth looking at. The 500 (or 600) is the best on-road/off-road combo unit I've seen. That is, it does both functions very well, while most only do one well (if they do more than one at all). It has full-function touchscreen street nav with complete detailed street maps of the entire U.S. But it also comes with 24k topo of the entire U.S. preloaded. (To top it off, if you have a boat, it also comes with blue water charts for oceans and lakes for the whole U.S. preloaded.) It's very customizable (I think you can store 1,000 waypoints), has all the trackback features, easy to use, waterproof, shock resistant, and has a nice big 5" screen (and an MP3 player, if that's a desire). It is bulky, though, and it does not have an internal battery.
Garmin Topo maps: Someone quoted a Garmin tech as saying they have both 24k and 100k topo maps available. More specifically, though, they only have 24k available for select state parks. The rest of the U.S. is 100k only.
Mounts: I have not tested a mount yet that did not work well enough to keep the nav system it came with securely attached to the windshield while bouncing offroad if mounted as instructed. I wouldn't worry about this too much in your decision-making process.
Touchscreens: How easy they are to use depends on the company. Garmin and Lowrance's touchscreens are generally great--easy to use on the fly without mistypes, etc. I find Magellan's harder to use than those two.
Timoss
88 fj62 TLCA