Another thing to keep in mind is non-hunters. Many areas of CO are popular with hikers and backpackers so while they don't kill elk they may move them around, or occupy nice campsites, etc. Something to consider as you look at areas and trailheads.
Go to colorado, are you bow or gun hunting? the fact you can take anterless is a big plus. Expect to see people in either state. Less in idaho, but there is less elk.
Remember Colorado the last week of September is closed. So you really only have one week after Muzzleloader to bow hunt, i dont know what it is in Idaho.
I'm planning on doing the OTC archery hunt somewhere in SW CO if I don't get drawn here in AZ. Seems like there are some great opportunities to hunt that region of the state. I also like the idea of being able to get an any sex elk tag.
SW Colorado is an awesome place to hunt, so there's lots of folks here. There's also a ton of folks here because it's a tourist attraction. However, there's nearly 2 million acres of forest here so it's not too hard to isolate yourself. Google and any other map just don't do this place justice. It's rugged and if you're not going up, you're going down. Even if you don't see any elk, it's just plain awesome to be out here.
I think here in Idaho we have a longer Archery Elk season with the opportunity every year to hunt in the peak of the Rut. Packing into the back country for some solitude sounds like a great idea but there's allot less Elk. The Wolves have done a pretty good job on the Elk herds in these Wilderness areas. While there are certainly Elk in these areas it seems most of the larger elk populations seem to be closer to civilization.
I live here and if I were you I'd do it a bit differently especially on a solo hunt. I'd look at the harvest statistics and %'s. That's a good place to start.
I'm from Idaho and we have a lot of Elk here! Lots of opportunities! But you'll have the same experience in Colorado as well, you'll just need to find that sweet spot. Elk hunting comes down to time in the mountains, you'll get an extra two day in Colorado, so I would do that!