I still think Fi-Base. I incline towards Gamma (ESI) based on that mixing of different elements of different eras, which I find not really Delta-ish, Deltas IMO are more into period authenticity, even if fantasy. This is not a very strong opinion, in the end even Tolkied mixed in several elements belonging to different historical periods as well, though it seems that in the case of Pratchett this fancy mix is the base idea.

I also know an ESI guy who was enjoying a quote from Discworld, it was something like resisting rain and wind and moving forward (not sure it is accurate).