1TB Fusion Drive is a rip-off, don't even think about wasting your money on that s***. It's basically a 1TB HDD with 24GB SSD (no, I didn't forget a zero, it's really 24GB). Your iMac will only boot fast and that's it; every other program will be as slow as it is with your Mac Mini mechanical drive.
2TB and 3TB Fusion Drives work really fine, as they keep the most used files on the SSD and the least used on the HDD (it's a little more complex than that but that's a helpful way to visualise it). For a normal folk, they work like a charm; not as amazing as pure SSD but they come very close.
I personally always go full SSD and keep my old files in an external drive. My type of work allows me to do that with no problem whatsoever but you must consider your need, usage habits and, of course, money.