Best corporate structure and registration location for a CO medical practice
If you plan to organize your medical practice as a limited liability entity you would probably need to opt for Professional LLC (PLLC) or Professional Corporation (PC). Also, if your medical practice is located in Colorado that's the state you would need to register it in.
As far as choice between LLC or corporation goes I would like to refer you to our comparison article: LLC vs. Corp. It is also a good idea to consult a CPA before choosing how your entity is going to be taxed. Just keep in mind that LLC can be elected to be taxed as partnership/sole proprietorship, or S-Corp, or C-Corp, and that, among other things, is the reason LLCs became so popular.