I am trying to make up my mind about a furling screacher on the pole versus a spinnaker . I would like to leave it in place virtually all the time but would like to understand more about the likely windage effect. (if I need to drop it when going upwind then it seems a bit pointless) Also what weight of sail cloth to go for, in the hope I can have something very light that might furl tighter and be less clumsy. Are there any Farrier sailors with experience of keeping a furled screacher more or permantly out there who might be able to advise me?

Depends on what you intend to do. For cruising I leave my screecher up most of the time. It comes down if I am going to weather in big chop beacuase of the pounding.

For doing bouy racing, if it is just W/L I leave it at the dock. You can point a bit higher with the finer entry of just the jib. If there is a chance of a reaching leg or a distance race I leave it attatched to the pole but drop it to the deck.

You cannot carry the screecher as low as the chute. If you think you want to furl your deep runner (chute) some people run a Code 0 on furling gear. I am quite sure that you shouldn't leave the Code 0 on the pole when not intending to use it due to UV exposure.

I don't think it's pointless. It's easier to furl the foresail and then drop the well behaved "tube", than to drop a spinnaker.
Afterwards, you may tie the "tube" to the net and leave it there, ready to hoist (or take out the halyard only). Or stow neatly furled in the cabin.
The spinnaker requires more work to set again, unless you bag it rigged. But then the cabin entry will be obstructed and there will be at least three loose lines on deck to worry about.

On the leaving it up virtually all the time thing - don't leave it up for extended periods when you're not sailing. My F-27 came with what was probably a very nice Smyth screacher that was completely and totally delaminated from being left up furled in the nice Florida sun.