Thanks for the replies so far! Actually, I could go either way, it doesn't matter as long as I complete the loop. This is just a soft itinerary, because as you both aptly pointed out, there's no way for me to know that I-25 would have been a dull ride. I'd LOVE to check out Durango, the particular route I have set up in that region is only set up that way so I could approach the Grand Canyons. Can you snake through the mountains in Colorado easily enough to Arizona instead? If I can I could bypass New Mexico altogether and visit Durango instead.
I also have no idea what to make my base city (meaning the city my plane lands at), but I'm leaning towards Denver as its the city I've wanted to spend maybe a couple of days sightseeing. I could land anywhere within the loop though. I have enough reward points from my credit card to take a free flight one way, so airfare isn't much of an issue.