Well, thats a different perspective, I was leaning towards 512 SSD, your heads-up is starting to confirm doubts about fusionI went fusion and I completely regret it. In many cases its not that much faster than the regular drive. I'd go 512 gig ssd or better if I purchased again. Your other option is buy the fusion drive now and add an external USB 3.0 or thunderbolt ssd as your main boot drive. A number of people have done it on here for far less than the 768gig upgrade from Apple and there is no performance loss over it being external.
Regards,
Gary