Hello! Have you seen these collars here?
www.kittycollars.co.uk they are really great collars - very safe (they will come off instead of hang your cat if it gets caught somewhere) plus because the info is on the tag, there is no need for an additional bulky bit
In any case, personally I prefer having my mobile number - so if one of my kitties is found, I can be contacted wherever rather than just having a message left at home. I also have my house name and street, so that if they are found wandering, people will know if they are local or not. I think the most important thing is a phone number as that is what people will instantly look for if they find a lost, injured, or (hopefully not!) a deceased cat.