I do not think Fusion is a good idea for the "pro users." I like consistency of file transfers, - read and writes. I want a pure SSD, with no "spill." Nothing more annoying than fast transfers..... and suddenly they become slow transfers. No thanks. SSD all the way. For me it was an easy decision. Thunderbolt Samsung 830 256GB, connected via Thunderbolt as a boot drive.
Reasoning?:
1.) Pure SSD speed. Thunderbolt SSD might be marginally slower than an internal SSD, but we're talking unnoticeable day to day. (I benched 390 MB reads and 340MB writes, as I recall, connected via Thunderbolt to my 2011 Mac mini. I expect similar (better?) results with a 2012 iMac when it shows up).
2.) Easy to remove and hide when going on vacation etc in case of break in. Just unplug it!
3.) Can then eject internal HDD because there's nothing more obnoxious than the sound of a spinning hard drive.
4.) Never having to wait for spinning disk to spin up = win.
I can see the logic in the Fusion disk idea, - but for me - no thanks. For the average user it's probably fine, but I like a bit more control over what goes where.
Is it annoying having something plugged in all the time (given iMacs are supposed to be AIO) and worrying that if it unplugs your system will crash?
Considering either your method or the 768Gb internal so trying to weight up the pros and cons