On March 2, RTCSNV is going to extend the Centennial Express to McCarran Airport. That'll effectively double the number of express buses from downtown to the airport (although Centennial Express does take about 15 minutes longer than the existing Westcliff Airport Express).

Additionally, both routes will now stop at both Terminal 1 and Terminal 3, eliminating the need for people flying in and out of T3 to catch the airport shuttle to T1.

Details here.