Dead Indian Pass is spectacular, and very quiet in terms of traffic. Services are somewhat limited. I rode it the other direction, and enjoyed it tremendously.
If you've never been to Yellowstone & the Tetons, though, that's pretty great too.
I think a great way to do it would be to use Dead Indian Pass, then from Cody head back west into YS, south to Tetons, then over Togwotee pass through Lander.
In CO, I would recommend going in between the routes you mention - From Dillon, head to Copper Mountain (on a bike path no less!) and climb Fremont Pass to Leadville (nice hostel, also camping), go south to Buena Vista, over Cottonwood pass to Gunnison. Your Gunnison & south route is fabulous, and make sure you do Wolf Creek Pass to Pagosa Springs.
If you end up using your more westerly route, make sure you take the back road on the north side of the Black Canyon of the Gunnison (from Hotchkiss, Hwy 92), rather than going all the way south to Montrose & taking 50 - 92 is spectacular and empty.
There are a number of ways to route through CO, most of them OK, some of them awesome. Your Breckenridge route would be my last choice, you spend too much time in South Park, which doesn't really feel like being in the mountains, and the traffic is a bit heavy, and very heavy on Hwy 285.
Gimme a shout if you want more details, I've ridden all the paved divides in CO.