The upgrade price is tempting, but if you go for the standard drive now, and buy the Vortex £ or whatever when it comes out, it means you still have the 500/750 drive that you can either put into an external or Optibay type arrangement.
At least that way you will have the best SSD when it comes out, and you will also make get to use the 500/750 which you wouldn't have if you went for the BTO upgrade.