My tuppence worth:
Looking at like-for-like, which is what the OP seems to be suggesting....
Microlights and LAA light aircraft both have very similar maintenance regimes: lots of owner work allowed.
With almost identical types - ie C42, Eurostar and Zenair 601s (OK, they have different wings) - the light aircraft version is usually significantly cheaper. People like buying the aircraft they trained on.
Otherwise, if you want four-stroke - and don't mind Lycoming/Contental type engines - then there are much cheaper (to buy) light aircraft. ie Condors. They'll use a bit more fuel though.
So there is a capital/revenue thing here: more cash upfront for a 912 microlight, less money on fuel and maintenance. Less cash upfront for a 912 light aircraft and - if an L or C engine - more fuel and maintenance.
You can hire a factory-built microlight. You can't hire a kit-built light aircraft (I think). You won't be able to hire a CofA aircraft after 8th April without an EASA licence
LAA even have a four-seat Jabiru that you can fly with an NPPL SSEA - so you could put someone in a back seat!