I was thinking a little more about this stuff, and I guess I realized that I'm not really as concerned with achieving maximum margin leverage than I am about managing risk. For instance I can sell 10x -685p for next week for a total of 5k premium, vs 17x 655/680 BPS for approximately the same premium. The puts will cost me ~250k margin reservation vs ~47k for the BPS. The puts only exhibit worse loss if the price goes below 635, otherwise they exhibit progressively much less loss as the price gets closer to 685. I could obviously thus sell 5 times as many BPS for the same margin and get 25k premium but my risk goes up to a much higher level than I am comfortable with. That said, I might be more comfortable with BPS if I sell some and manage them and see how it goes, but I'm not as brave as I might want to be right now.