I've hunted numerous times in Colorado and Idaho. Both have OTC tags for certain units. The one thing I experienced with CO was a LOT of hunting pressure. I'm sure you can get away from crowds if you know the right area, but the rifle seasons tend to be circuses. I hunt off my back and have found that I need to go 5-8 miles from a road in CO, versus 1-2 miles in ID. ID has some really rough terriain and that could be the reason.
Therefore, I'd say ID. I define success as more than taking an animal....that's certainly important, but solitude is much more. I think you'll find more elk in CO, just more people too.
my 2 cents