@hereinaz I agree, in terrain where you're going rock to rock or walking on loose rock, softer boots can be exhausting and miserable. That was true for me in Arizona even without a heavy pack. I didn't understand that to be the condition of things across the entirety of the American west.
@Lytro My apologies, we both could have worded things differently, and I didn't have to respond the way I did.
To clarify, I have not hunted out west, as I stated earlier. I've carried stupid loads up, down, and long in California and I pack animals out back home as needed (which isn't the Rockies).
My experience with stiff boots is the opposite of a lot of people's. I'm fine with them going up, and even happy with them if it's especially steep. Coming back down, they are torture for me. The longer I'm in a pair of stiff boots, on any terrain, the more I wish I had opted for the more flexible boots.
I haven't noticed a difference in how my feet feel in stiff versus soft boots in situations with heavy weight versus without heavy weight. Maybe I don't carry enough weight now for it to matter? I generally draw a line at about 75 pounds these days, other than a couple of times strapping a whole whitetail to a pack in the last few years for a short pack out. Maybe above 75 pounds it starts to matter whether you're wearing stiff boots or flexible boots? I don't know. The heaviest loads I've ever carried were in soft boots and at the time I didn't even know there was such a thing as a stiff boot. I was younger then, but 10-15 miles weren't a problem. Over 20, my feet were going to be trashed. Maybe stiff boots would have treated me better.
Ultimately, OP will have to decide the boot thing for himself. There are factors other than, and in addition to, where he's going that affect boot selection.