So I have run extensively in both as well as the VP 4%. I was very excited when I saw the SP as I figured it would be lighter (it is), and the heel counter less rigid ( it is). The shoe is basically the Fly with the 4% upper!
Here is what I found. While the upper is from the 4% in design which is lighter and more flexible in the heel, the material used is not good. It's plastic like, not breathable, and for some reason, they run much smaller, almost an entire size! The ride is basically the same as the Fly, and it feels a touch better since it's lighter, but the upper sucks so much I had to send them back as it just didn't fit right and wasn't breathable. I'm convinced now that the SP is a lifestyle shoe and was not intended for serious runners. Now if it had the same actual upper material as the 4%, it would be a winner! In fact, my dream version of the Zoom Fly 2 would be just that, the same lunarlon midsole with the exact upper from the 4%. The shoes would be lighter and fit better and still have the wider supportive platform.
With that being said, the Fly doesn't even come close to the VP 4%! Damn, best shoe ever. Nike just needs to figure out their production issue and get it going. My only change on v2 of that shoe would be to make it slightly less curved, which would give it a tad more support, while adding a touch of weight, which wouldn't matter all that much.
Hope that helps!