The premium is just for the name as you suggest, as you get the same silver content and purity in both bars. Keep in mind that although you pay more for a perth mint bar you are very likely to get the same premium back when selling in the future as perth mint carries more trust and is well known amongst even international investors.
I actually just came across a new site called www.metalsbazaar.com in which you can connect with others who want to buy and sell silver thus cutting out the middle man which should mean paying less of a premium. Seems like a good idea but doesn't have many members yet
