ABS Keycaps
Cost less to produce and can achieve slightly crisper legend printing, but tend to develop a shiny, worn look on frequently-touched keys within months of regular use.
PBT Keycaps
Cost more but resist shine and wear far better over time, and are typically double-shot molded (legend molded through the material rather than printed on top) for longer-lasting legibility — now the expected standard on premium-tier keyboards.