I highly recommend the break-away collars from k-9 collars.

http://www.k-9collars.com/products.php?type=Cat

They will embroider you pets name and/or your phone number on the collar. I'd buy a couple of them because Pepper loses them from time-to-time and we usually find them weeks afterwards. But, Mina will be too little for even their smallest collars for a while.