Maybe I'm missing something but what is the point of the 256GB option? Pay $300 more for only 128Gb extra flash compared to fusion, lose 768GB storage, and miss out on the automatic aspect of frequently accessed data being managed onto the SSD.
I might have chosen that option, because I have ~190GB total space used on my Fusion drive, so all of it would be accessible at SSD speeds. Since I have my Aperture and iTunes libraries on a NAS, it's not likely that my storage usage will go up significantly in the future. I have 3 external SSD drives, and when I include the price of the required enclosure, the $300 price for the 256GB internal SSD is reasonable.