The blue Z is a little less stable. Purple Z seems to be less stable in all molds, but I have yet to find a purple Pred to try it.
I have two max weight ESP Avengers that have flat tops that are pigs. I have a first Run Avenger in Z that might as well be a Predator. I have two max weight purple Z Avengers that are flippier than my ESP Flashes in the same weight. I have an optic green max weight Avenger that is nearly identical to the First Run Avenger.
I am not complaining here, in fact I LIKE the variation within the mold. Once you know what to look for, you can get all the flights from one mold.
I too am interested to find "more straight" (easiest to flip up flat) color/plastic Preds. Finding overstable Preds is easy, look for dome near the rim.
The straightest Preds (easiest to flip up flat) in ESP/Z that I have tried so far are as follows:
Lime Green ESP
...and stay way from the dome at the rim. Look for a smooth top near the rim.
My lt. blue first run Z Avengers are STOOOPID overstable, they may as well be Firechickens. Whereas the orange ones are nice controllable, somewhat overstable drivers. I've also got two Ice clears and they are total opposites. I've noticed that if there is a definitive "step" in the flightplate with a little more dome, it's regular. If it's a smooth flightplate with a little less dome it gets too overstable.
That's avengers only though, every mold is different.