I'm not a big fan of the spray foam insulation kits mainly because they are expensive. But if you're going to be tearing a bathroom down to the studs, I would probably spend the money for a closed cell spray foam kit.
Bathrooms can be tricky places to insulate properly. Especially if you have plumbing on exterior walls. Electrical outlets, wires, plumbing and ductwork can make it difficult to get the rockwool or fiberglass to fit exactly right. Any voids or mistakes will cost you on those cold winter days.
Spray foam makes it quick and easy to get around the obstacles and does a great job of air sealing. It's kind of an insurance policy for frozen pipes if you do it right.
Don't forget to insulate the rim joist beneath the floor. Cold bathroom floors are not a girls best friend