“I know (I think I know) that in the UK microlights are limited to 450kg.”
The above micro could be on a PFA permit or a BMAA permit. No difference in restrictions, no Night or IFR. 450kg is MTOW of a micro, stall speed etc also comes into the definition. The MCR01 ULC (a Micro) has a biger wing in order to get the stall speed down to the Micro limit. This makes the Micro heavier and slower than the Club version, for exactly the same operating costs.
“I don't see an obvious difference between operating costs of the two.”
Depends what you compare. A PFA Permit aircraft can have 4 seats and 260 hp, but as stated above the VLA versions of micros have the same running costs but higher useful load and sometimes better performance as well.
“One curious difference is that one can fly a ML on an NPPL and take it to France, but one can't do that with a Permit or a CofA type without permission.”
I understand that the French have given permission for an NPPL (SEP) to fly in France, but I am waiting final conformation. This is how the Micro PPL’s first got permission to fly in France. The French then got fed up with issuing separate permission so allowed a blanket approval.
Rod1