Too many factors that can play a part to give a clear answer to that question. What I observe is that most players have a lot to gain from just working more on the mechanics of their break, than what material the break shaft is. After the mechanics, I would say the tip type and tip shape is key. I'll gladly sacrifice a tiny bit of power for more control, so a hard leather tip over a phenolic or plastic type material. I see way to many break cues with a rounded tip, similar to a playing tip or sometimes with an even more pronounced curve. Quarter shape is perfect imo.
After that it's all the usual stuff, the ferrule material and design, the taper of the shaft and the tennon design.
I think the Mezz deep impact shaft has a perfect taper and the super short G10 ferrule has no flex to it, so you don't loose any power.