Hi, Tom: Either the 705 or the 800 can be used for navigation purposes. The 800 is touch screen (not a big deal, but I like it). The 800 also has more pages of information; however, if you are using it strictly for navigation, there is no real advantage to the extra pages. Both track every conceivable data you would ever need for training or races or events or whatever. Most of that information is not necessary for a tour, but some of it is pretty good to have. Both have heart monitors (and I like the HM on the 800 better than the 705). As someone pointed out above, they are not primarily made for navigating across country; however, I was able to take Adv. Cycling's maps and using www.mapmyride.com
, create the route for NAVIGATION PURPOSES. It took me about an hour to create one section (400 miles).
The problem is waypoints aren't created on MapMyRide; however, with a smart phone that's not a big deal. I looked at MapSource (as recommended by AC); however, the mapping in Map Source is very klutzy. I don't recommend it at all...nor does Garmin for navigating across country on regular U.S. hwys/rds, etc.
The BIG ADVANTAGE to using either Edge is you don't have to look at the printed maps as you ride. I would still want to carry them with me just in case you run into a road outage; however, Garmin (both versions) will take you to the next town or around whatever you encounter.
Back to your original question: 705 vs. 800. I used the 705 for 3 years and found it to be a great unit and I have been using the 800 for 5 months. I would get the 800 because it is later technology and seems very reliable.
Both units (the 705 more often than the 800) will indicate you are "off course" when you are not. You need to be aware of that because it might "scare" you at first; however, if you are on the right road, it will indicate "course found" pretty quickly.
Let me know you have any other questions.