The choice is definitely redwood (already purchased), and while the
deck (in my opinion) will look nicer with no spacing between the
planks, I don't want to risk damaging them from shrinking and
expanding in the Colorado weather. I have heard just as many "no
reason to space, redwood shrinks/expands very little" as I have
"definitely space, you will have problems if you don't" answers to the
question.

So is there anyone that lives in Colorado or a similar climate that
has experienced this specifically and can give some "been there, done
that" advice?

thanks,
Erik