Baker is about a 2.5 hour drive from Seattle, Whistler adds another hour and a half. That's a lot of driving for a day trip!
The thing about Baker in April, they are closed Monday through Thursday. That often results in epic snow-fests on Fridays! OTOH, Whistler is a couple thousand feet higher in altitude, and that can mean a lot in the Spring.
Plus, Whistler has better groomers.