I think that your questions rely heavily upon the players perception as the best way to find answers for 2-4 are to try them both out and draw your own conclusions..
I am not sure about the taper but I believe they are both conical, someone will correct me if I am wrong.
I have tried them both and personally prefer the OB-2. I see very little deflection, the same was true for the Z-2 though.
I don't know how to measure this but I feel that the stroke is where you get the spin from. I do however find it easier to draw my rock with the OB-2. Once again, this is my perception and am sure that the tip has a lot to do with this too. I am using the standard tip on the OB-2, I believe it is an Everest, someone correct me if I'm wrong here.
Stiffness, I like the OB-2 in this department too. It just feels more solid to me. I have played with most LD shafts out there and went back to a conventional shaft for the past 2 years until recently purchasing an OB-2. I love it.. It feel solid, has a stiff feeling to me, although I've read others whom don't feel the same way. Just my opinion.
You really need to try em both out and draw your own conclusions, that is just my opinion. One of my biggest reasons for going with and OB shaft is their customer service. They REALLY stand behind their product with fantastic support. They also do a lot for the industry but Predator does too.
YMMV, Joey