Help me decide bt SSD & Fusion

If you can afford the SSD, I would get the SSD and then add an external drive if you needed expansion. The larger Fusion drives have 128GB of SSD attached, and the operating system is pretty smart about migrating frequently used blocks of data onto the SSD, but as you point out, spinning disks are much more prone to failure. Replacing an internal disk in a new iMac is a huge pain to do yourself, and probably $$$ to get done by a professional. By constrast, a USB 3 disk (or even better, a Thunderbolt disk drive) is somewhat future-proof in that you can expand it as you need to, grow into a RAID system later if you want, and easily replace it if it fails.

BackBlaze produces an awesome report regularly on disk failure rates. I would speculate that a frequently used computer disk drive will last on average 5 years, but that's a bell curve -- a few will fail in the second year and a few will run 10 years.

I was having the same question, I finally choose the 1TB SSD. One thing that made me choose this config is the fact that with Fusion drive Windows 10 would be installed on HDD rather than SSD making it slower. Another thing to bear in mind, if at any point you want to play 3D games, they will run faster with SSD.