I've been reading a lot on expo. I do have a Garmin but want a secondary map source with more detailed maps. I have a basic Garmin nuvi so I'm not sure exactly what I can and can't do with it.
Your choices:
1) GM Navigation - which is pretty much useless off-road (though, occasionally even a blind squirrel finds a nut). Worst part about GM navigation is gm-only software so you can't download maps
2) dashtop navigation - includes any of the ones you see in cars on the street. You can download offroad maps then make the unit use that software, but it requires programming and is not user friendly (I've done this with my garmins, but I don't use other gps units mostly because I can only really afford just one platform
3) handheld - in my opinion the best option because getting software for wherever you're going is pretty easy, down side is most of them operate off batteries and will not allow you to plug into your car... this is true with most, if not all Garmin handhelds (I have at least a dozen of different makes). One of the other reasons, if you need the GPS outside of the car, having a portable one is a nice option.
4) aftermarket radio navigation - my Kenwood stereo allows the use of memory cards and mapping software, it works okay, but at this point, it's not been reliable - so I'm not ready to recommend it to anyone
5) professional navigation like Loran - of course, these units start at $2500 - so cost may be an issue along with where to mount it... GM felt we needed passenger air bags (but my wife is already there - haha), so that nice location to put it has some issues (you'd break the navigation is the air bag went off
- which of course won't go off unless someone is in the seat, or some dog)
I don't add phones, I've used everything suggested and more - and the biggest liability is the battery. They last maybe 40 minutes before going completely dead. Worse, their antennas are designed for more populous places - trees affect them way too much, and too often you don't even notice that it's location is plus or minus several miles...