Marts
I have never trained at either but having flown from commercial airports I would strongly advise avoiding it, due to the very reason you said. MAN is a busy airport, you'll be waiting for ages to get off the deck and will end up orbiting on the way back waiting for a gap.
I have flown with Liverpool Flying school which was a good outfit, reasonably priced, however that was 2 years ago and I know that LPL is becoming increasingly busy too. Nowhere near that as MAN though and it's only 20 mins from MAN.
Your best bet is trying the private flying forum for this as there are a lot of guys there very up to speed with schools in that area.
SK