Which is generally considered superior for garbage disposal grind components, galvanized steel or stainless steel?
As noted in the other answer, the manufacturer warranty is probably a much better indicator of quality than anything else. There are many variables that affect durability, even just of the cutting blades, never mind the rest of the appliance, so it's not likely to be useful to look just as the composition of the blades.
That said, all else being equal I would expect stainless steel to be superior to galvanized. The latter involves a sacrificial layer of zinc protecting the underlying steel. When this layer is compromised, initially the zinc near the damage will corrode preferentially, protecting the steel, but eventually the nearby zinc will be consumed and the steel itself will begin to corrode.
On the other hand, stainless steel is inherently corrosion-resistant (nearly corrosion-proof). It has a thin layer of oxide, but if that layer is damaged, instead of it exposing underlying steel to corrosion, the steel itself will reform the protective oxide layer. The efficacy of this process depends on the quality of the stainless steel, but it is in general a much more reliable protective effect than galvanization because it naturally "heals" itself, rather than naturally degrading over time.
Note, however, that with both types of materials there are different grades of steel that can be used, different grades of processing to make the final product, and different degrees of tolerances and quality-control. It's possible to make a very high-quality galvanized steel product and a very low-quality stainless steel product. Even when all the other variables are controlled for and equal, without knowing exactly the quality of the galvanized and stainless steel in each product you're comparing, you can't say whether one is necessarily better than the other.
Which again brings us back to the manufacturer warranty, the closest thing to a reliable indicator of quality.