Big gliders!
Basicly the both aircraft are big gliders and the GRP structure is little understood by most UK maintenance companies who have a "send it to the factory" attitude when you have a little bit of hangar rash, this results in high costs and long down time that could be avoided by selecting the right maintenance company for both maintenance and repair.
Most of the good GRP repair companys in the UK are in the glider business and so not approved to work on powerd aircraft, some have a Part M subpart F and could do some powerd aircraft work, one or two have full EASA 145 approval and those are the people that you need to talk to if you want to keep the cost of airframe ownership down.
The engine situation is in the hands of the factory's both engines have had a few problems but these are being overcome, Austro has the advantage of being slightly behind Theilert and so has seen some of the problems that Theilert have had, on the other hand the Theilert engine has much more time in the air (most of it in UAV's) and soon will be in a position to extend some of the maintenance items (the clutch change should soon go to 600 hours).
From a control point of view the Thielert is a less complicated engine and the weight is much lower than the Austro, I would say that the Theilert weight advantage balances the Austro power advantage.
The jury is still out on the Austro on the issue if cold inflight restart and what to do if the re-start fails, but that is an issue that could do a thread by it's self.
At the moment on the engine issue I can see no clear advantage one way or the other between the two Deisel engines, the only thing I can say is that with fuel costs going up both of these engines will become more attractive.