Because the AX is type approved for training, has a UK source of spares, a coherent flight manual, and stall characteristics that are compliant with the airworthiness standard? (Or the X'Air all but the type approval). Plus you're dealing with the BMAA, not the LAA - which in my experience is massively more user friendly if you want any mods done.
I don't believe the OP mentioned that he had any aspirations to train on the AX?
As for spares, Rans owners I have known have never had any issue gaining access to spares for the design.
Coherent flight manual and compliant stall characteristics? These are simple aircraft, the flight manuals for which could be accommodated on the back of a fag packet. As for stall characteristics, surely acceptance into the UK microlight category would only be granted if the the stall characteristics are compliant?
The Rans has a proper engine mount and steel structure around the important bit. The occupants.
Gets my vote.