I hope I am not too late in responding to your query..
From the sound of it, it looks like you have similar options in all three universities. So, maybe you should look at other factors like climate and job market in addition to research. I know for a fact that Boulder and NC have good job markets after graduation. Boulder generally has a number of research institutes and being located there will certainly be an advantage strategically. Also, NC has three universities in that region closely located to each other. So, you will be able to leverage greatly on the network there (especially that of Duke, has a great env. school). As for climate, I personally love NC's climate in general. Winters in Denver can be a little too harsh. That being said, both Boulder and NC are beautiful places with a lot of hiking options. Arizona is popular for hiking as well, but the summers are too extreme for my liking. And as far as professional networks in Arizona are concerned, I am not too convinced they have a great research network. The university is too "commercially" run for my liking (although it is a public university). But, that's my personal opinion and people may disagree with me..
In any case, you seem to have great choices ahead..good luck and congratulations!