No question, SkyRide hands down. Route AF:

http://www.rtd-denver.com/skyRide/

I paid 6$ one way last Monday, DEN airport to downtown. There is a stop at the light rail station just east of downtown (30th or something, forget the exact location). Free transfer from the RTD bus to light rail, can get you just about anywhere that you need. I think the next stop is the Denver bus center or something like that.

I asked to be dropped at 20th and California (between stops downtown), just before they make a right hand turn near Broadway, driver said no problem. I then had a 1 block walk to Hertz local edition, rent car at 1/2 the price of airport pickup. Priceless.

Make sure you have exact change for the SkyRide bus, they have an information desk inside the DEN west main terminal, but they don't sell tickets there. You have to pay cash on the bus, into a small cash receptacle machine, and the driver won't have change. Just carry fives and ones for RTD payment and you'll be fine. It might take a little bit longer than SuperShuttle (due to the few extra remote parking lot stops before downtown), but the comfort of a full-sized motorcoach over SS vans, plus the obvious cost savings, is a winner.