Mitchel....if you're planning to reach TDF,
AND come back....you have the option of down one way, back the other.
As Patrick indicates if you want to see lots of other bikes and people, and great coastal scenes, use the WC route.
If you want space, few bikes, few people, and great mountain scenes, the CDT or Great Divide Route is the way to go.
The great thing about the GDR is that few people have seen it - Patrick's right that the route can use highway work arounds, depending on weather, road constructions, wash outs and the like. You can do 90-95 % of it on Forestry Road and Dirt surfaces if you want and are able. When wet, parts of the route are virtually impassable. You can also adjust as you see fit in response to your daily desires/situation. There's a huge range of options.
The reports from most people who've ridden the route seem to suggest it as one of the best in NA - it isn't always technically demanding; it isn't super exotic (in the 'across Russia on a moped' sense) but it is one of the 2 or 3 routes in NA that gives you lots of options, some degree of adventure, links you up with incredible history, and gives you both great journey and destination.
You may have seen this report but if you haven't it'll give you a few pics - and you can link to others' reports at the bottom of that page.
GDR Ride
I used Michael McCoy's book on the route but also bought lots of Forest Service Maps enroute to add detail and variations.
Best of luck in your decision making!!!
Stephen
Red Deer