If I remember correctly, on B&H Photo, the 256GB machine is the same price as the 2TB machine. That's just too big of a drop off, space wise.
I think you need to think of your long-term
storage needs. If you're going to need external drives anyway
then supplementing a modestly-sized SSD with externals isn't going to be such a big deal - and you keep all that nasty spinning rust outside of your iMac.
For some people, 512GB (maybe even 256GB) is going to be all they ever need (you can get an awful lot of WP documents and spreadsheets in 512GB) - for others, well, if you can fill 1TB, you're probably going to fill 2TB as well. If you're doing serious video you'll probably go through external HDs as if they were floppies...
If you want to store your bulky media collection, then a NAS that can be accessed from set-top-boxes or tablets might make more sense than having to keep your iMac on all the time.
...and, of course, you'll need backups.
I opted for the 2TB Fusion drive, because I was tired of dealing with external drives with my MBP,
Well, a laptop is a slightly different kettle of fish: unless its a totally sessile "desktop replacement" that never leaves the desk, there's a stronger argument for having as much internal storage as possible so you don't have to faff with plugging, unplugging and carrying around external drives.