A possible solution for you would be a Fusion Drive plus an external SSD. That is what I'm looking at. I'd have the OS and most things on the Fusion drive and use a small (probably 256GB) SSD to hold things that I want guaranteed on the SSD. This would include Aperture libraries with the photos being referenced and on the fusion drive. It would also be a couple of games that can lag when they need to access data while playing.
Of course if Apple drops the price a lot for their SSDs, I might look at a 512GB internal SSD for the system disk and an external hard drive for photos and the iTunes library.