I think it also depends on your skin type. It could be different now, but when I was looking to purchase my pearl necklace a couple of years ago. Tiffany's pearls had a silver tone to them and Mikimoto's pearls have the more common/ traditional rose tone. For example, choosing a wedding dress; not everyone looks good in the same shade of the color white. I leaned towards Mikimoto's pearls as they had the rose tone that look so good on me.
BTW, I ended up purchasing from American Pearl and went with the 7.5-8 as it would give me something to grow old with. American Pearl also has alot of interesting information on their site about what to look for in pearls, it impressed me enough to buy from them and I have not been disappointed.
As a funny aside, I wore my necklace into Tiffany's and the lady asked if it was a Tiffany's and I told her no it was not. She then pulled out the best single strand in the 7.5-8 that she had and there was really no comparasion. She was shocked to see that what I had was far superior to what she had and also shocked with my knowledge. I really do not think that she expected that.
***I do not or have I ever worked for Tiffany's, Mikimoto's, or American Pearl. But, I would be more than happy to answer any questions. Good Luck and thinking of this as a timeless piece makes it an easy purchase!