I would recommend that you get a CVT Insight. They deal with steep hills much better, because the transmission will use exactly the gear ratio that it needs. The CVT is also much better at managing the battery charge, so you'll be less likely to deplete the battery on mountain roads.
I never drove my Insight in the Rocky Mountains before. However, I drove it from San Francisco to Lake Tahoe once. I believe it was something like an 8000 rise, and the Insight did very well. The battery charge dropped to about 1/3 and quickly recharged. I even found myself passing people going 75 MPH uphill (and there was a fair amount of power left over for acceleration). I felt like I could have gone 80 or 90 uphill. Don't get me wrong, the Insight is no sports car. However, I was surprised how well it did in the mountains.
The CVT does some pretty smart battery management. When it's almost fully charged, then the battery only recharges when you take your foot off the gas. However, if the charge is around 2/3's, then the Insight gets more aggresive at charging, and starts to charge when you are cruising at a constant speed. If the charge is even lower, perhaps around 1/3, then it charges the battery even while driving uphill, and only stops charging when you are significantly accelerating.
The end result is that it's pretty hard to deplete the battery. Most uphill climbs consist of steep sections mixed with flat or less-steep sections. If the battery is low, then you'll be able to pick up some charging on the less steep sections. I was surprised how well it works, and have never been able to deplete my battery.
Disclaimer: As I said, I haven't done any Rocky Mountain driving in this car. I have a hunch that driving east through Vail Pass might deplete your battery. I seem to remember a 10 mile stretch where it's all 7% uphill. Fortuantely, the car still works when the battery is depleted. It just has less power.
Good luck with your decision. I hope you buy an Insight!
