There is no "harder or softer" - PBT is all the same in that regard. I'm typing right now on a HHKB and the PBT caps are almost, but not quite, smooth. The texture is very light. I have some PBT from Signature Plastics which are very smooth, and I have a couple of sets from Enjoy PBT which have a nice texture to them. I have a Cherry keyboard with genuine Cherry brand PBT caps and they're pretty smooth.
I don't really have a preference. The textured ones have a nice feel, but the smooth ones have a nice feel, too. Just different. The texture might be a deal breaker for some people but not for me.
The ones with heavy texture can be a bear to clean, though. I use an ultrasonic cleaner so it's not a problem but without that tool the smoother caps are a lot easier to clean.
[EDIT]
I forgot about my Ducky G2 Pro PBT. Those caps have a pretty heavy texture as well.
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