Colorado has laws that give monopolies on the infrastructure to whoever owns it. But in order to get around anti-trust laws (or for some other cynical reason), so in your case, there is really only one provider, but they share their infrastructure with their competition and throttle them to nothing. So your options are "whatever comcast offers" and "whatever comcast decides CenturyLink can offer"
I don't understand. Comcast has their own infrastructure: a bunch of coax in the ground. Century Link (previously known as: QWest, USWest, and Ma Bell) has their own: Cat-3 coppter in the ground.
DSL/Telephone copper line has physical limits to the bandwidth it can provide. It can't do anything close to gigabit and plummets very quickly with distance from the telco box.
