Most fish spatulas have stainless steel flippers, but a handful of manufacturers make a nylon version, for use on nonstick pans. I tested one of these, but it confirmed my suspicions: A plastic spatula can't be made thin enough to function well as a fish spatula. It has almost no flexibility, and is too thick to slide under delicate items, like tilapia fillets, without damaging them. For a fish spatula to do what it's supposed to do, it needs to be stainless steel. The corollary, of course, is that metal spatulas are not a good choice for most nonstick cookware, given their propensity to scratch the pan's coating. A plastic spatula is a better bet when cooking with nonstick, and, since a nonstick surface allows food to slide around more easily anyway, the spatula's thicker construction will be less of a problem.
The material of the handle may also matter if you have a dishwasher. Spatulas with wood handles aren't dishwasher-safe, so, if you're averse to the idea of hand-washing anything, go for one with a poly handle. That said, it's pretty quick and painless to hand-wash a spatula, and some may prefer the feel and grip of a wood handle (not to mention the fact that it won't melt if left in contact with the edge of a hot pan).