I've had good experience (call it luck if you want) with 12 springs in combination with the 3.8 lever. That would be my personal choice. I think that with 15 springs, the rev/high clutch application may be delayed a split second too long causing a mild engine flare on the 2-3 shift. This is bad because the flare generates heat within the clutch before it is fully applied, thus shortening it's life span. Go with 12 & I think you will be fine.