Being a recent owner of a GTB, and never having driven a GTS, I am not in a position to answer your questions.
The only point I can make here is that the value of the cars seem to vary based on the country you are in. I noticed open cars are often very well perceived in the US, but in Japan, where I live, there is a clear premium for closed cars. This may be due to many things, the fact you won't open your car very much during the year, because it will either be too hot or too cold, the fact people don't like to be seen that much, the fact GTS are indeed supposed not to handle as good as GTB... This last point will hopefully be covered in posts to follow.
My personal opinion is that if you're seeking a car that will likely not lose value, and that may (I said MAY, not will! ;-) ) increase in value in the coming years, then the Vetroresina is the way to go, which means GTB is the only option. You will also end up with the lighter car of the whole breed, and possibly the most powerful. I say possibly only because it seems like Ferrari changed the way they calculate power between the early carbed cars and the later Quatrovalvole. If they didn't, there is a good chance the 240hp of the later cars would have been pretty close to the 255hp of the early ones. Of course, I am not counting the 328 in this comparison. But even there, I would be surprised the 328 will ever reach the same value as a Vetroresina 308.
Not sure this helps in any way, but that is what I could say that would remotely answer a small part of your questions, although most probably in a biased way!
PS: I didn't read Vitalone's statement before posting this, but I see we have pretty close views!