As everyone has pointed out 1) Paper tickets are better if you need to be changed to another airline. Other than that, since you get an iternary (proof) and a confirm number (more proof)with a E-Ticket, a paper ticket has several disadvantages. 1) It can be lost. It then is costly and time consuming to replace. 2) Many airlines offer BIG incentives to use an E-Ticket that you don't get for paper tickets. 3) Paper tickets must be mailed or must be picked up and paid for at some possibly distant location.
In the case of CO if you don't have a really great need for the time savings of a paper ticket in case of a cancelled flight, you can't afford not to take the E-Ticket for the miles. A 50% bonus is not to be ignored!!!