I wouldn't view AEA starting LGW-MAD as such a negative for Manchester. Lets be honest: in the current economic climate it was really rather unlikely that they'd have done MAN anyway regardless of any plans that they might have elsewhere.
As I see it, if they can make a decent go of the LGW-MAD then this ought really play into Manchester's favour as I'd view it a pretty clear cut choice as to what ought to be the destination for a second route into the UK from MAD once the Gatwick is running well.