i5 IMac or i3 IMac?

Both have a 1TB HHD

Not impressive. A big +1 on the solid-state drive (SSD) option. We've see easily thousands of reports of slow iMacs with mechanical hard drives since the 21.5-inch 2012 models were released with small and slow laptop-class drives.

Even the faster desktop-class HDDs commonly used in 27-inch iMacs seem to be struggling with newer MacOS versions. Theoretical processor speed effects on performance are bupkis compared to the documented bottlenecks caused by old-school mech hard drives. I'd take an i3 with a factory SSD over an i5 with a mech hard drive any day.

Apple's Fusion (hybrid) drive are acceptably faster when healthy but the number of reports here of the more complex Fusions having issues disturbs me. Admittedly, many of those complaints were caused by people trying to do things not needed, like reinstall the OS, but I has seen a number that started slowing within any ill-advised intervention by the user.

I have a small collection of iMac drive score data. The fastest mechanical HDD, in a 27-inch iMac, scored write/read speeds of 170/155MB/sec.

The slowest reasonable after-market and DIY option, the USB3 external drive with a 6G 2.5-inch SSD, will do 400/400MB/sec.

Healthy Fusion Drives start as about 500/1000MB/sec and can go as high as 800/2000.

I just tested my 2017 iMac 5K quad-core i7 with the optional factory SSD:

See why I recommend the factory SSD? That makes a computer react fast. 🏎 It is so good it is worth giving up Starbucks for a month or two to cover the extra cost.

Neither, you should look at an Apple refurbished iMac with a minimum of 16GB of RAM and the largest SSD you can afford. Both choices you have selected are major downgrades.

Take rkaufmann87's advice and ensure that it is also at minimum, a Core i5 CPU. Review the system requirements of non-Apple applications that you will be using for their processor, RAM memory, and graphics processing requirements.

The i3.