I would say the C42 is more robust for training, as evidenced by the fact that just about every 3 axis microlight school uses it.
The Eurostar can certainly be used for training and several schools do, but the low wing and thus the access via the wing, the perspex canopy etc... mean that it is likely to get shabby quicker than a C42.
As for operating costs I doubt there's anything in it as they both use the same 80 hp Rotax, same fuel burn/maintenance etc...