I tried both years ago, and found very little difference between the two on my old reringed '73 440. 12.60s with the SD, and 12.50s with the RPM, MPH was exactly the same between the two...the only time it would break 106MPH is when it had wheelspin. If it hooked up, it went 106 every time no matter what I did.

RPM felt perkier at small throttle openings when driving it easy. SD was also fine, but not quite as responsive to a light touch on the throttle. I had a 4 grand converter at the time, which should cover up any differences, but it was still noticeable.

I think I liked the rpm a little better on that combo but the difference was small, can't really go wrong with either one.