I've had experience of both operations at Nethersludge having flown there on and off for almost 10 years.
To answer your questions:-
The airfield is well drained and is flyable for most of the year but occasionally suffers waterlogging and/or subsequent damage after prolonged periods of rain in winter with occasional closure or reduced operations (no landings/touch & go's during circuits).
Wind directions and local features occasionally stop flying at Netherthorpe when other airfields continue to fly.
The Aerobats that Phoenix use have a little more grunt than other 150/152's and so have better take off performance - useful out of Netherthorpe.
They are also well maintained and equipped.
For your 'membership' of SAC you get a good clubhouse, if this is something that you feel a need for it'll cost you extra.
There are no membership charges at Phoenix but the tea is free and there's always someone to talk to.
Both operations have employed 'hour builders' instructors on occasions - they've always been professional.
I have more experience of Phoenix's instructors and cannot recommend them highly enough - very professional, very experienced and always willing to help.
I have very limited experience of SAC instructors, I know one or two and they too would be recommended.
For my money - Phoenix - and they organise fly out days in the summer where students are welcome.
I never regretted doing my PPL & IMC at Phoenix and my night qual. with a Phoenix instructor.
BTW - As far as I know, SAC don't own the airfield they lease it and are the operator.
And there's been a few Cessna shaped holes in the hedges!
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