I'm a novice on snowplows, but can answer your hydralics.
Force from a cylinder = piston area X pressure; since you need to subtract the rod's area from the retraction [pull] side of a cylinder the extention [push] can produce more force. The irony is that [especially on small rod sizes and/or long travels], you're subject to collumn buckling [bent rods], before tensile fracture [pulling it apart]. I think another thread had some great pix of that effect on a BH cylinder. Of course considering the travel you're looking at on a plow blade, it really negates any concern for rod failure.
Since it's often cheaper to add size than number of components, I don't think the cost of the cylinder itself is your answer, but for the way it is integrated into the plow some size restriction may be important and may have a cost benefit that way as well.
When I think of blades, it's one at each end [single or double acting] for angle adjustment. But it sure sounds like you're referring to two singles used as a double, which, barring some dimensional restriction, is something I've never seen. /forums/images/graemlins/crazy.gif