As far as one big gift card or a variety, it really wouldn't matter to me. For my son's preschool class one mom collected several different cards and then made an arrangement of them (We made the scrapbooks too, big hit). Don't forget that movie gift cards are great too, especially if your child's teacher is a Disney fan. You can treat her to Pirates!
The bottom line is that as a teacher any recognition of my hard work is wonderful. I love personal things like scrapbooks, etc. I also love the dollar store figurines that came from students at the first school I taught at. It was a very low income school, and they were so proud to give them to me.