I have followed this discussion with interest since I am the designer/builder of the Stealth F16, one of the boats mentioned.
Most of the posters are right, and are actually agreeing with each other, but unfortunately the argument has become a personal one and the answers to your questions are a bit lost in the mud that's flying around.
Your question about whether spinnakers produce dificulties in certain types of boats.
Well I think that it is too simplistic to put cats into these two camps of Hoppers and ploughers, my experience is that all the cats around today are pretty easy to sail with spinnakers. Some respond better than others but I suggest that the main difference is in the developement that has gone into the boat in the first place so that it is more a question of refinement rather than, the effect that the hull shape has.
next on the point of weather helm or lee helm
It is a simple fact of life that if you stick up a spinnaker you move the centre of effort forwards, now if you have a balanced boat to begin with you will end up with lee helm, if the boat is balanced with the kite up, there must have been weather helm before you put it up. You cannot have a balanced boat both ways unless you mave the daggerboards forward when you put the kite up.
Some boats, the I20 is one, have very balanced rudders, so that you cannot tell if there is weather or lee helm, but it is still there.
Bill is right in that his design will make a more neutral helm when the kite is up, but he failed to mention that his design must have more weather helm when the kite is down, its just that he has balanced rudders so you can't feel it.
The real question is whether or not any of this is a problem, and the answer is that it is not, just select a boat that has been designed by a good company, and develeloped with the spinnaker on it and you will have no problems with any of this.
Any of the boats you are looking at will do in this case
