Many BlueNile stones are available to others in the trade; you could ask your jeweller to call the stone in assuring him you will buy one or the other, or you can pay for the BN stone on your credit card, and send it back if you feel the extra money is not worth it, or that you'd prefer to buy from your local jeweller.
On Fluorescence: no preference - unless the fluorescent diamond trades at a discount (and then I'd get the discount). Fluorescence may help the diamond look whiter in sunlight, but a] there are relatively few opportunities for someone to look at a diamond through the side in sunlight, and b] they will look a lot whiter in all lighting situations if they are well cut, so I'd spend on that.
For example, look at the larger centre stones in this 1920s clip:
reasonably white, right? Well, not if you look at them from the side...
where they can be seen for the K-L stones they are. Both pictures are unretouched, taken with the same camera a few seconds apart in natural light. Also note how the leftmost stone shows colour in both images, but the others don't.
Edited by davidelevi, 02 November 2010 - 01:06 PM.