I live in Denver and can say I hate I-25 all the way from the Wyoming border to south of Colorado Springs because of the traffic! Traffic through or around Denver can get heavy and I-25 between Castle Rock and Colorado Springs can grind to a halt at any time if there's an accident. Speed limit there is 75mph and drivers want to do either 65 or 90 in the left lane. This frequently causes bad wrecks in this section. Raton Pass at the New Mexico border is a steep and windy climb out of Trinidad where you might get slowed down behind a truck and need to slog it out.
Google Maps suggests this route: https://www.google.com/maps/dir/Mile...3!2d33.4483771
I've driven the stretch from Casper WY to Craig CO and it's doable but services are few and far in between. Unless you're into sight seeing as part of the journey, I'd think your better bet would be your I-15 route.