Regardless of which of the evils one chooses, be sure that you back up regularly, those files that you cannot reproduce or it would be a pain in the tush to do. Do it to an external source. I have the G-technology drives, one is a 4TB and the other 6TB Thunderbolt 3. Now the 6 is solely for my Time Machine and a few choice other things, the 4 is what everything backs up to.
I also have an off site backup on everything, CrashPlan. I assume there are others and I am not locked into this one, so like the Red-Green "Man's prayer," I can change.
However, back up your files right away, incase you decide to stay with OS Sierra or other, or go with the new one. If you have to reformat your drive, well you don't want to lose your data. Apps/ Programs can be reloaded, but the data , well you get the direction I am going.
I have a fusion drive and yes, I am going to wait until the thundering herd of others make the upgrade. I would rather wait, but I know me, for the AFS for fusion down the road.