When I took level I of the CFA exam in December of 2008, The CFA institute did not publish/send study materials. As a result, a candidate had to choose between Stalla and Schweser review materials for the exam. Since I used Becker Review for the CPA exam, I decided to use Stalla for the CFA exam (Becker and Stalla are owned by the same parent company). I bought the Stalla review set, viewed all the lectures, and studied based on their materials. However, two months before the exam, I talked extensively with a friend about his experience taking level I. My friend advised against using Stalla because he thought Stalla focused too much on the details and that the practice questions from his review did not accurately reflect the questions he faced on exam day. He recommended using Schweser. So I purchased Schweser notes on Craigslist and supplemented my Stalla materials with Schweser.
At the time, my conclusion was that Stalla's video lectures were much better than Schweser's video lectures. While Schweser's written study materials and questions were much better than Stalla's. However, since the CFA Institute now sends candidates study materials, I recommend using the CFA Institute materials as your main study source then supplementing that with either Stalla or Schweser based on your study preference/habit. For me, that meant immersing myself in the CFA Institute material then referencing Schweser notes on occasion, completing both CFA institute and Schweser practice problems, and doing as many CFA institute and Schweser practice exams as possible.
It is virtually impossible to comprehensively use two different study sources because there is simply too much material to cover - so figure out what works best with you and stick with it!