The Eurostar at PIK is the group A version - essentially identical to the micolight version other than a fuel pump and paperwork. Its on a permit to fly - costs are generally the same as if it was the micolight variant (they sometimes have shares available - if you're interested, contact them via Prestwick Flying Club, although they do vet other group members carefully).
However as others say - most aircraft are one thing or another, you can't easily switch between certification schemes. Also if you are paying someone to do the maintenance anyway - the costs end up pretty similar.
As you say - PIK doesn't allow microlights (after an incident a few years ago when, I understand, a group of microlight owners made a claim against the airport, after their flex wings were damaged by wind from a 747 engine). Landing fees for based club aircraft are much lower (under £9) than what visitors (or what aircraft based at the flight centre) pay.
There are advantages and disadvantages of all types and locations.
XA - of course - some of your based aircraft fly into Prestwick for fuel... people complain about Strathavan not being the smoothest in places and grass in a Scottish winter can get a bit muddy. There are also ownership limitations.
Cumbernauld is quite high up so is more likely to be clouded in.
It would all depend on what sort of flying you want to do, where you live, what access to an aircraft you want, whether you're happy doing work on the aircraft yourself...