I would be leery of leaving saftey pins in that long especially in humid weather. I would go with the tag gun. I've used mine on different occasions and like it. YOu don't just push it thru the 3 layers, you actually do the same motion as pinning. Down thru 3 layers then back up. I would try it on a sample sandwich and see how you like it.
Maybe someone else will have personal experience on leaving the pins in an can give you a definate answer.