Plastic boots are overkill for the Sierra (possible exception of the Mendel ice couloir) in May. The only place in the US I've ever climbed with my Koflach's was on Rainier in spring, but in retrospect they were too hot and heavy.
markjwpcp: For Whitney, stick with a more traditional boot like the Asolo's you mentioned. You can go to the heavy side if you anticipate more spring or even winter Sierra climbing, but still should have no problem staying under your $250 bogey. If you're planning a trip to the Himalaya, Karakoram, Ellesworth, Andes, Alaska Range, etc., then spring for the plastic.