In reply to Tigger:
I used Westfield Fasteners for all my T-nuts. I bought zinc coated for my outdoor wall (it is covered and never in contact with water) but from memory they do have stainless. I don't know whether they have screw-ins though.
Having used the four pronged type for my wall, I'd concur and recommend screw-in Tees indeed. I have not actually had any spinners at all but hammering 240 Tees on plywood is loud enough to be utterly antisocial and surprisingly faffy. Any newly used holes on my wall need to be retighened a couple of times before the nut is sat fully as well, and that would not be an issue with screw-ins.