I recently purchased the latest 27" iMac to replace my 7-year-old Mac Pro. Because of how I use my Mac, I had no trouble deciding between Fusion Drive or all Flash Drive (SSD).
One of my most frequently used apps is Aperture (I'm a photographer). I have nearly 50,000 images in Aperture. Of those, very few of the master images were imported into the Aperture Library file. I store the master images on a secondary hard drive. But even with only about 5% of the master images getting imported into the Aperture Library file, said Library file has swollen to nearly 95GB in size.
The SSD portion of a Fusion Drive is 128GB. The OS figures out which apps and data you use the most and moves that stuff from the spinning drive to the SSD. However, since parts of the OS also need to be on the SSD (for maximum performance), I'm sure the available space (on the Fusion SSD) for your data and apps is a fair bit less than 128GB.
With a 95GB Aperture Library file, I was concerned that it was too big to fit on the SSD portion of a Fusion Drive. And it's a file that gets accessed a LOT when I'm using Aperture.
Aperture had grown INCREDIBLY slow on my 7-year-old Mac Pro, despite the fact that (on that computer) I had the app and the Library file on a fairly fast RAID 0 drive. One of my primary goals in getting a new Mac was to speed up Aperture!
The new iMac with 1TB SSD (Flash Drive) did the trick! Now, 100% of the app and 100% of the Library file are on fast SSD (700+ MBps). Aperture FLIES on the new iMac. Importing images and editing them is a JOY on the new Mac. Fast, fast, FAST!
I am confident that Fusion Drive would have also been faster than my old Mac Pro. But I'm equally confident a Fusion Drive would NOT have been as fast as this full SSD machine!
It came down to spending $600 extra (the difference between 1TB Fusion and 1TB SSD). In my case, it was $600 WELL spent!
This sucker is FAST!!
Mark