It all depends on your preferences. I tend to personally value miles at 1.5 to 1.66 cents per (valuation is itself a subject of hot debate, though). Merchandise is usually keyed to list price or close and isn't a deal, and car rental certs are maybe penny-a-mile. I haven't been thrilled with hotel conversion rates, either.

So I usually go with air travel. But if you have trouble finding the time for trips or getting a round total for the award, it may be different. Also note that CO is one of the most complained-about airlines for award availablility. You'll do fine, though, as long as you don't want Europe, Hawaii, or Asia.