I can think of one argument why UEFA might rule first:CAS haven't even put the hearing of City's pre-emptive appeal on their list yet and I'd be very surprised if UEFA announced any "verdict" before that is settled.
If UEFA find City guilty then they can whiten their image, and do nothing if CAS later rule in our favour.
If CAS rule in our favour first, then UEFA can't even clean their image and be seen to have tried their best.