I don't have a fusion drive, so can't give a valid opinion. But seems like a good compromise.
I have a 2 TB spinning drive in my iMac.
I have a 240 GB SSD in my MacBook.
MacBook definitely faster. No surprise!
If 512 GB is more than enough space, then go SSD option.
Video uses up a lot of space.
Whilst I'm all for SSDs, I just thought I'd pitch in here. The Fusion drive has very similar read speeds to an SSD, alongside reasonably fast write speeds.
Basically a Fusion drive uses the 128GB SSD as a middle-man. When you save a 20mb photo, it'll save to the SSD super-fast, then later move it to the spinning drive if you don't view it again for a long time, effectively hiding the slowness from the user. It's a clever idea to keep storage prices down and compete with the PCs offering 2-3 TB of storage (average consumers will just go for bigger numbers - "Flash storage" means nothing to them) but, ultimately, it's a temporary hack until Flash storage comes down further in price.
I went for the 512GB SSD in the 5K iMac because, for me, external (and eventually cloud) storage is the way forward. I don't want an iMac with all my stuff on - that's too limiting. I want a server in my home with all my stuff on, and my iMac to simply be one way of accessing the content - same for my iPhone, iPads, MBPs, etc.. I need a bit of space for device specific things like apps, games, device-specific working files, etc, but 512GB more than covers that, again,
for me.
Also worth noting that if you use Bootcamp on a Fusion Drive Mac, you're entirely relying on the spinning disk whilst in Windows. You're now in the situation where you've spent £2,500 on a mid-range desktop and have a bottleneck from the late 1990s. Not cool. The other downside to Fusion Drives is that you have a spinning disk in frequent use in your Mac - this will increase noise and heat vs. an SSD.
It's your money, so it's your decision. Just be aware of the drawbacks of each option. A Fusion Drive is the
only solution for someone who needs 3TB of onboard storage in their iMac. It's also a
considerable performance jump over the regular HDD in the 2013 model. The SSD option provides even better performance, but at a significant cost and unfavourable storage sizes.
As for your question of 2013 vs. 5K iMac. No-one here should be answering that for you. If you feel that a 5K screen, a slightly better processor and slightly faster graphics performance is worth $500 to
you, then go 5K. If it's not, go 2013. To help contextualise $500, though, remember that a
5K screen alone will set you back $2,500 at the moment - the same price of a 5K iMac.
Think about what you need, don't just post randomly specced iMacs on here and ask for a yes/no from people, because only when you get to the
maximum spec will you get a unanimous 'yes'
