As for the whole yellowing debate, *shows off chemist badge*, this is the way it works
.ABS yellows. Trust me, it REALLY does. I have several ABS boards that haven't yellowed at all, but really, they ALL will. PBT is virtually (but not completely) immune to yellowing. You'd have to put it in direct sunlight for an extended period of time for it to become anything but absolutely virgin white.
ABS dissolves quite dramatically in acetone, PTB doesn't. You can use this to determine which of the two it is if you're not sure. Just do it on an inconspicuous area.
PBT is harder to mould than ABS however, and as such the spacebar isn't easy to make come out properly with PBT. This is why spacebars on boards with PBT caps often still have an ABS spacebar. This is easy to see because it'll almost invariably be much yellower than the PBT caps. This property also makes PBT very hard to doubleshot; only a few examples of this exist (reportedly on the Ortek MCK-101, for instance). PBT is often dyesubbed instead.
Whether you prefer PBT dyesub or ABS doubleshot is entirely a matter of opinion. On ABS doubleshots, the printing is of higher quality, on PBT dyesub, the plastic is. I prefer ABS myself, as I like the super high quality look of the legends, and even thin ABS caps sound noticeably bassier on keys than thick PBT ones do. That said, I can see why people like thick PBT too.
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