I agree with Karen, definitely allocate at least 2 days at Denali. I suggest avoiding those so-called tour buses, Denali's own shuttle buses are essentially the same. The main difference is that "tour" bus driver must provide a verbal descriptions of what you are looking at and tell stories where as park shuttle bus drivers aren't require to do so. I believe tour buses also provide a lunch box whereas you have to bring your own drinks and food on park buses.
Park's shuttle bus is better if you plan on getting off the bus and hike a bit on your own. The next bus with available sits will pick you up.
We picked parks' shuttle bus with hiking on mind, but the weather was wet, windy, and a bit cold.