In my humble opinion (please I am not trying to offend anyone, but this is just my opinion based on a lot of research and economics):
A diamond is a terrible investment in of itself (not fungible, not actually rare, can be lab created, etc etc...all bad qualities to have for an investment). So whether you buy a huge stone from a non-branded jewelry store or a smaller store in a branded store like Tiffany's or HW or Cartier, you are still paying for an essentially worthless piece of stone that will drop in value by 65% the moment you leave the store. So, in my opinion, if the girl wants Tiffany's, then why not? Ultimately, the diamonds don't actually hold their value the way they are perceived to hold value. So even if people pay premium at Tiffany's, they are paying premium at a local jewelry store as well, since the diamonds that consumers buy for jewelry are so marked up in price anyways. That's just my opinion...in all honesty, I would rather buy a Tiffany's ring, because since diamonds aren't really an investment, at least TIffany is.
I agree with that. You buy a ring because you like that particular one, not because it is a great value. Better just keep your money on your savings account then, or buy a gold bar.
That said, I don't really like the design of the classic Tiffany's ring or copies of that ring, it seems quite boring to me, but I do think it's a 'work of art', there is a reason it is copied so much, even though the design seems really simple. I do like some other Tiffany designs, especially some Paloma Picasso pieces.
Furthermore in the country I live in, there are very few Tiffany stores, and I they are located on the most luxurious shopping streets in the city. Right in between the Louis Vuitton, Hermes, Chanel, Cartier, etc etc. It's regarded a highly luxurious and exclusive jeweller, but it's not that well known as in America I guess. It's astonishes me that someone called Tiffany a 'mall-jeweler''? Is Tiffany really everywhere in the USA?