A few thoughts:
You may need time to acclimate/adjust to the altitude. As sweet as it sounds...I would caution against a route that popped you off the plane and had you spinning to a high-altitude trail/pass immediately. As a 25 year resident in Ski Country..i've had numerous friends arrive to ski only to soon be in a heap with headache/nausea and worse due to pushing it to hard/coming up too fast in altitude from the lowlands...alcohol consumption... physical exertion too soon after arrival/etc. If this happens I guarantee you that riding a bike will be out of the question.
September weather can go both ways but exteneded cold/snow isn't the norm. Days are often warm and nights, *crisp*. Frost usually arrives the first week or so of the month..especially above 10K.
Crested Butte is a mountain biking heaven, for sure. You do, however, take a chance *if* the weather were to turn off cold/wet....and there isn't any low ground for miles if that were to happen to bail out to without a long ride West.
I've vote Durango...maybe Moab. While it will be hotter in moab..the 4000ft elevation is less likely to have a negative effect...and in this area there's everything from the "slickrock trail" to the Manti-La-Sal forest where you can traverse from the desert to a high-alpine environment @ 11K or so with a view that will knock your socks off.
Another place fast gaining popularity is Fruita..near Grand Junction. Koekepelli trail...
Durango is higher....but you could head south there if the weather closed in on the peaks or the altitude proves too much/etc.
look for local links...bike shops//etc...some of which will have local trail info/etc.
good luck!
