I think Bolland's art always looks beautiful, but that is a pretty static image in his unused cover and I can see why the powers that be went for something more dynamic.

I agree, the original Ditko cover is much better.  The only thing I can think of is that maybe Stan didn't like the fact that Spider-Man looks like he's losing the fight in the original? 

I've often wondered if there was another rejected cover between the original and the finished version.  It seems odd that Kirby and Ditko are collaborating on the published cover.  Usually when Kirby drew Spider-Man it was because of expediency... ie, he was there in the office and Ditko wasn't.  So how did this cover come about?  If Ditko was available to draw the Enforcers, why didn't he draw the entire cover?   I'm thinking perhaps Ditko did draw a second version of the cover by himself, but Stan still wasn't satisfied so he later had Kirby redraw the Spidey figure.