Something else to consider... cashless tolls on the tollway from the airport to I-25 the car rental companies will include a toll-pass for a per day fee, which may be a rip off if you only use the tollway once or twice, but have the car for any length of time. Some companies will mail you a bill for the tolls if you don't take the transponder, along with an "administrative fee" You can bypass the tollway, but it will often add quite a bit oftime to a DIA to springs trip.
https://www.e-470.com/Pages/HowE470Works/Rental-Cars.aspx
https://www.e-470.com/Pages/HowE470Works/Rental-Cars.aspx