Depends on the budget and the experience you have. If you have had lots of track time in a shifter, and your budget allows it – go with Stock Moto. There are some good shifterkart drivers in Colorado, both in Denver and in Grand Junction.
If you don’t have a lot of time in a kart, I would suggest starting out in the LO206. It will teach you racecraft and how to be smooth. That can transition well into a faster kart, if you decide to move up the speed charts after time in a LO206.
Check out http://www.billetperformancekarting.com/BPK.html as they do both shifterkarts and LO206. DD2 is not very popular in the country, but it is also an option if you are looking to go Rotax racing.