The simplest way to think about this:
HDD: This costs less and gives a lot of storage, but it is a little slow.
SDD: This gives you comparatively less storage, costs a lot more, but it is really very fast!
Fusion: This doesn't cost less, but it doesn't even cost much, it gives a lot of storage and is quite fast!!
So,
If you use a lots and lots of heavy applications at once, or maybe you have a lots and lots of huge applications on your mac or even games, then you may go with SSD,
If you use your mac simply for watching a lots of videos, pictures, browsing the internet and all the other super basic stuff or let's say you have a huge collection of movies, then, HDD is the way to go, because it will store a lot of data and you really don't need heavy performance on all that...
And finally, if you are into stuff like video editing, or photos, then Fusion Drive is a great option, as all the apps that you will use for your editing and other stuff will use the SDD and give you a great performance, whereas all the videos, pictures etc. that you will actually be editing will be stored in the HDD!! Or even if you do all the three above i.e you watch a lot of movies, or you use a huge number of heavy applications, or if you are into editing then the fusion drive is excellent!! And if you still can't decide, then the fusion drive is an excellent option, I promise, this thing actually performs great!! This thing is technically for everyone!!
And don't worry about stability, as an iMac is a desktop, not a laptop, so it will not be exposed to very tough conditions too much and will most probably sit and shine on a beautiful desk at your house/workplace!!
...I hope this helps!!
~Cheers
-Priyaanshu
