We actually stayed at both resorts last month and the reasons you love Four Seasons will influence your decision. Both are wonderful resorts for different reasons. Why we loved Kierland- proximity to lots of good restaurants, grocery stores, shopping, etc., fun activities for the kids poolside with lots of other kids to play with, love the golf course views, free shuttle available to the hotel, Kierland Commons, Scottsdale Quarter.
Cons- Kids activity center was very expensive and therefore, we did not use their service, housekeeping tidy service midweek only, pool was crowded even in July, sometimes hard to find a decent parking space near the villa
Four Seasons- kids club has activities from 9-5 every day free of charge, pool area was quieter and wonderful service, closer parking to villas, daily full housekeeping service, Keurig coffee service replenished daily, huge rooms
Cons- remote location, desert landscaping with very little greenery
Things may be different in busier months but both resorts are really great. It all depends on your interests and family.