It depends on the disc and what you want from it. In the predator, I like Z because I want that thing to stay overstable longer, as Z is more durable. In the surge, I definately like the ESP version better. Better glide and straighter/longer. It held a line longer before fading off. I didn't like the Z surge at all.
I like both, but use them differently. I found that I was overdriving some holes using the Force, and reverted back to older drivers. I try to only use as much disc as a hole needs. A Flash is probably my choice for line drive shots, or shots which must hold an anhyzer lo line for 95% of its flight, say 350 ft speed.
It's easier for people with slower throws to achieve distance with a slower disc....a really fast disc will hyzer out sooner and probably produces an off-line skip at the end of it's flight. I guess that's all right if you have room to work with, but it's easier to throw a Flash straight at it.
I liked the more overstable Z-Surges. I've got some Goldenrod gold w/ Red stamp Z Surges which respond a lot like a really fast Predator. Almost have to release them on anhyzer angle to get long flights, but I know that they will come back at the end, reliably.