I'd suggest dropping the weight a down to allow you to complete 2 sets of 15-20 with good form. Be sure to focus more on pulling with your abs, and try to leave your back/arms/hips/etc. out of the equation. There should be a very small range of motion in your movement, just like when you're doing a normal crunch. (I'm guessing that pulling with the hip flexors might be what's causing your back pains.)
As for swiss ball crunches, I don't know much about them to be honest. I guess they would offer a larger range of motion(Letting you lean back more.), so I assume that it would increase productivity much like standing on something to let your heels drop below parallel on calf raises.
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