Recommendation - i5 or extra RAM?

Good call on the factory SSD. Having used one in my 2017 iMac for almost two years, I would no longer let a mechanical or Fusion drive darken my door. This SSD screams!

Remember, with a 21.5-inch iMac, the RAM you order today will be your "forever" RAM. Apple won't add RAM later and, due to the difficulty in accessing the RAM slots (you have to practically gut the computer), many Apple Authorized Service Providers and independents won't do the work either. A local one who will do it says it takes at least two hours labor at US$75/hour to perform this task. That's more than the cost of the RAM modules.

The 2020 iMacs are not yet included in the benchmark data in the MacTracker database but this is how the i3 and the i5 from 2019 compared:

That's a massive improvement simply in choosing the i5 that year.

...photo editing and videos. Very little video editing, but it would be nice to be able to do that without it being painfully slow.

Given the performance difference from 2019, I say the i5 AND no less than 16GB RAM for the needs you've described. Don't give up on the SSD dream though. You'll regret it.

i3 with 16GB RAM. Ram isn't user upgradable and photo editing can be memory hungry.

Still, the i5 will get the job done faster. Have you considered getting the i5 with 16GB and a 256GB SSD... and then storing photos/videos on an external 1TB USB-C SSD?

Buy a minimum of an i5 and I would also recommend upgrading it's RAM it 16 GB or if you can stretch it to 32GB. Also include the 1TB SSD and then you will have a pretty competent computer. Don't scrimp, if you need to save longer then please do so. Also, remember Apple has some pretty good financing options. If you are in the USA they offer 3% off is you have an Apple Card and if you open a Barclay account you can get up to 18 month no interest financing if your credit is good.

I just ordered 2 ea. 2020 27" iMacs and while I had saved enough to pay cash I opted to take advantage of the Barclay 18 month no interest offer. I will pay monthly for the next 18 months to use Barclay's money instead of my own.

For light video editing and future proofing, the minimum you should go for is the i5, 1T SSD and 16GB.