Generally by the look of them, the Zierer ones tend to have a bit more of a traditional look to them compared to the P&B ones. Having ridden both, I find the Zierer ones to be a little bit smoother in operation. However out of all the companies that build them, I'd say those 2 are the best. The biggest difference is the base of the ride, the Zierer ones tend to be more of an octagon shape and the others tend to be longer.
I will admit though, Volare at Luna Park is a very very good looking unit and certainly has a lot of that classic charm either way so I'm hoping that Dreamworld's version is of similar quality so I'm happy either way.
Zierer Model
Preston and Barberi one