Re: Which one? Estes Park CO or Big Sky MT.
Estes Park, RMNP, is something else if you love the high country lakes and solitude. Spent a week there 2 summers ago and it was fantastic. Fished a bunch of high country lakes and the Big Thompson in RMNP. Used Kirk's Fly Shop for advice and they seemed to know their stuff.
Can't speak for MT as I haven't been there yet but it's certainly on my list!