If you want built in auto-routing (ie, pick a point or enter an address and it will tell you turn by turn how to get there from where you are), go with the GPS V or the StreetPilot III. If you don't need that, and want a larger screen, go with the 176C. If you want to save $100 or so, get the B&W 176.
Comparisons: http://www.cycoactive.com/gps/gps_compare.html
I have the 176C and it works very well. Definately go with your instincts on the hardwire. It'll eat up batteries quickly. (The 176/176C comes with the hardwire kit, since it's meant for a "fixed-mount" marine applicaiton.)
I also used a RAM mount, attached to my right clip-on pinch bolt.
http://www.cbrxx.org/cbrxx/images/misc/gpsmount1.jpg
http://www.cbrxx.org/cbrxx/images/misc/gpsmount2.jpg
http://www.cbrxx.org/cbrxx/images/misc/gpsmount3.jpg
http://www.cbrxx.org/cbrxx/images/misc/newcockpit.jpg
They all come with a decent basemap of the major US roads. Plan on getting at least a 64meg cartridge for it, and the Garmin MapSource software so you can load detailed roads, gas stations, rest areas, etc. With 64megs I can get the whole state of NC and a good portion of the surrounding states. Depending on the density of your area, your needs will vary.