if i could only pick 1, i'd go with whichever had the longest season.
In New Mexico, that would be black powder. Not sure about Colorado, but if....strike that...when I go back to hunt New Mexico, it'll be with my Hawken caplock. Not only is the season a LOT longer, but the weather is better. First snows normally come along around rifle season. Of course, that can be a good thing, too, for finding tracks. But, I'm a southern boy and I'm gettin' too old to be cold.
In New Mexico at the time, haven't looked in 20 years, you could only hunt ONE season in ONE compartment. You could not hunt rifle AN primitive seasons in the same year. Might not apply anywhere, but there and they might have changed that, don't know.
Gettin' inside iron sight muzzleloader range is a might harder than getting inside 7mm Rem Mag range, too. I prefer to have to stalk. That's the fun part of hunting out there, planning a stalk. Don't work a lot of the times, but it's fun to try. It ain't hard to spot good bucks, but those things have ears like radar and you have to be aware of the wind direction, then you see what cover you can use. It can be tough when you gotta get inside 100 yards.
Great fun.
Be aware that if you chose a BP for BP season, I'm pretty sure in Colorado it must be a sidelock using caps or flint, no inlines and no optics. You're doing it like Jim Bridger did it.
When I rifle hunted, I used a 7mm Rem Mag. The STW was out and all the rage back then, but I decided 7mm brass was going to be more available. I didn't know at the time whether the STW craze was going to pass or not, but I just didn't wanna go there.
The 7 has great exterior ballistics with high BC bullets.