I am going full SSD... combined with my 8TB Thunderbolt Pegasus R4. My biggest reason is because I want to control what is on the SSD.
Example:
My #1 performance sensitive app is Aperture 3. My library is 350 GB and has hundreds of thousands of files.
With SSD: 100% of my A3 library can be on the SSD
With Fusion: The A3 application, and some of my library can be on the SSD, but I have no control.
In either case... the application itself can launch quickly. However, since my A3 library cannot fit in the SSD portion of the Fusion drive... as I scroll through 100's of thousands of pictures... my disk will be thrashing away trying populate the thumbnails and preview photos from disk... which in turn will cause stuttering or temporarily blank frames. With the 100% SSD... everything will be resident on SSD (at my direction)... and from experience... I know that it will fly.
Bottom line: I can control what lives on the SSD... and I can put everything else on the Pegasus. Candidates for the external array (RAID 10) will be my iTunes library (not performance sensitive) and 100% of my videos.
Everything (including both the SSD and Pegasus) will be double backed up via TM and Crashplan+.
/Jim