There is a long current thread on the forums about this issue - I can't remember exactly, but I think they determined that it was very difficult to get a "fast" one - you actually have to boot the unit and look in the "About this Mac" menu for specific model numbers for the SSD.
The truth is that the speed difference is a bit academic, and really isn't noticeable in actual use, so my opinion is that it's not worth losing any sleep over.