As you already know, the MH Trango is an expedition class shelter which I personally feel is overkill for the Canyonlands anytime of the year. I would stick with your REI Arete & leave the warmth containment to your sleeping bag unless, of course, you only have a 45 degree bag & are looking for a reason to buy the Trango. If this is the case, I would purchase a warmer sleeping bag first & save yourself some $ in the process.