I went through the same thing with SS/SL when I began lifting. I felt like I stalled after awhile and started questioning what I was doing. Truth of the matter was I just wasn't eating enough. In my opinion, it is a great way to make a lot of strides quickly as a beginner, but make sure you're getting enough fuel for the workouts in your diet.
If you do go with 531, I'd recommend picking up the Beyond 531 book and exploring some of the ways to get more volume in the workouts.