Do you have to stay at the downtown Westin? Why not stay at the airport Westin? I agree with DenverBrian; take the A line to the downtown. It runs every 15 mins when you arrive (until about 7pm or so; then every 30 mins). From Union Station, I would take Uber, pedi cab, connect to another light rail line or even walk (depending on the weather). The walk from Union Station to the Pepsi Center is only about 10 mins and 1/2 mile. If you take the A Line back to the airport that night, it will leave every 30 mins (top and bottom of the hour). Plus the $9 ticket is good for a round trip if used on the same day.