other things to consider:
driving time of 3h40m is not usual! (chsommerhh, sure you weren't flying?
) True, leaving in the early morning is a must. 4 hr-4.5 hours is more likely, particularly if you make a pit stop (in Barstow).
Weather in mid-May won't be overly hot as you drive through the desert, which is a good thing!
Flying time is just about 1 hr. So once you take into account arriving early at the airport, waiting for luggage and getting a rental car (or taxi to the hotel), it is just about even for time.
I prefer to drive as I like going through the desert. Plus I like to have a car while in Vegas as taxi's are expensive and take forever because they take the most congested routes.