Personally, I don't think she should give up Spike. After all, wasn't he the one who had been with Twilight throughout her years at Crystal Prep? Surely Spike and Twilight had some time together. Fluttershy, on the other hoof, seems to really like Spike. But that's just because of her particular interest in animals. I don't see why Twilight should give Spike away to Fluttershy. After all, won't it hurt Spike's feelings?