Steve is thinking about getting the 27" iMac with Retina 5K display, and he's wondering about the Fusion drive. Leo says he has one, and that screen is amazing. But the problem is storage. And there are only two choices -- the Fusion drive, a 3TB spinning drive that works in concert with an 128GB SSD drive, and an SSD.
The Fusion drive is not as fast an SSD, and only slightly faster than a hard drive. He could just go with the SSD and then attach an external Thunderbolt 2 drive. They're expensive, but just as fast as an internal drive. If speed is of the essence, the pure SSD is his best choice.