Hi Lisa
Always a personal opinion based on personal experience.
I got one of the early Fusion iMacs being, at that time, the only realistic choice. I found that it worked fine, but experienced the following:
In order to utilise the faster SSD part which is small, storage wise, I was advised by Apple, at the time, to move my home folder to the SSD. Exact details escape my memory, but things went okay till they didn't and I had to send the iMac to Apple under warranty.
It never worked properly from then on. On 2 or 3 occasions the "Fusion" broke and Apple had to talk me through the process of uniting them, via Terminal - which is scary if, like me, I have zero Terminal skills.
Oh, BTW, when you do TM backups, they work as you'd expect, but in fact they create 2 BUs - SSD and Spinner - and I only found this out when I got my next iMac and had to do 2 transfers from TM to the new iMac with a single spinner. More problems. Apple to the rescue.
As your purchase would be out of warranty, I'd advise against Fusion. Now, my experience may be a one-off. But I'd go for the SSD as it will be way faster and you wouldn't have to worry about potential Fusion issues.
NB. A very personal opinion.
Ian