Go for pure SSD.
iMac 27-inch is a huge desktop, it's not going anywhere from your desk. So go buy some large external drive to store your media. Even 1TB won't be enough over time. If you have budget by then get some speedy Thunderbolt disk packs. You not going to missed much speed. Just don't keep so much junk in your internal drive, 256GB do fine.

The reason not to go for Fusion is the main part of the fuse is conventional rotational HDD, which will fail & slow down over time. Not to say the SSD won't fail, but they are proven more reliable when you compare with HDD. We still can't figure out why Apple still offering them in 2019.