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ABS is a denser plastic than PBT, but it does have lower hardness...
Sorry, but no. PBT is both denser and harder wearing than ABS. POM is denser than both: [http://www.stelray.com/reference-tables.html
GMK keycaps seem to take longer to wear than SP DCS. At least it takes more time and use to wear the "matte" surface of GMK caps smooth vs the "matte" surface of SP ABS caps. Could be the surface texture used, the specific formulation of ABS used or a combination of both.
PBT and POM are both harder wearing and will take longer to go smooth, but all keycaps will wear in time. IBM Model M and F keycaps are PBT, but you can find many shiny examples out there.
Personally, I like the shine, shows that the board gets used
If you want to keep the shine away as long as possible, get PBT caps with a fairly rough molded surface texture, like old thick Cherry PBT sets, one of Ivan's new PBT group buy sets, a Leopold PBT set from eBay or a PLUM / Noppoo set. The Poker II caps are also thick PBT with a matte surface, but they're OEM profile.