SSD is faster and more reliable. Max size from Apple is 1T (but can be upgraded to 2T with readily available components).
Fusion has an HDD (spinning hard drive). These wear out, generate heat which cooks all components from the inside.
If there's a real need for speed, get a 2015 and replace the guts with a blade. Otherwise, you can do the same but install a SATA SSD. Not as fast as the blade but still cool and reliable.
What you need depends on the use. Both my wife and I make our livings behind our iMacs.
My wife edits books and teaches school. She doesn't have much music or many photos on her machine. An i5 with a 512G SSD is all the iMac she is likely to ever need. Doesn't matter if SATA or PCIe because nothing she does requires speed except booting in the morning. I like reliability and hate fan noise so the spinner was pulled from her iMac years ago (when that was a lot more expensive than it is now). She has a 2011 that meets all her needs but wants a Retina — working on it...
I work in the music industry and take care of Macs for clients. I need a minimum of 2T for my boot drive and hate to wait for projects to load. A 2015 i7 with a 2T NVMe PCIe 3x4 blade is the slowest iMac that meets my needs and I need another 4T drive for my streaming VIs (virtual instruments) as 2T isn't big enough. I also have external SSDs that I use to test applications, compatibility, OS and beta etc.