Well, basically the Mini Core i3 8th generation processor is a 7th generation Core i5 with turbo boost permanently on. The i5 8th generation chip on the Mini has 2 additional cores over the i5's 7th generation and a slower clock speed. The single core speed on the i3 isn't that vastly off than the i5 single core speed. To get a better speed jump on both single core and multi-core performance, the i7 is a better choice from a performance and longevity stand point. The only thing that is in the i5's favour is the 2 additional cores and you need to know an application that can take advantage of those 2 additional cores. Now, if you are buying something else other than the Mini 2018/2020, then I would recommend having those 2 additional cores to help in rendering HD footage. However, the current Mini 2018/2020 comes with what is called a T2 chip. Aside from its security and other connectivity functions it do, it also does something else. It is also a hardware accelerator that helps encode AND decode HD and 4K movies at much faster speeds. In fact, it can decode and encode h.264/h.265 movies with Quicksync (an intel hardware encoder/decoder for HD/4K movies) that even a single Core i5 chip or even a Core i7 can not match. The T2 chip is much faster. It even smokes my Mac Pro until I use my RX580 GPU to encode and decode the movies. That is the MOST time consuming task of any movie production. It is to decode the movie h.264/h.265 (interframe) into usable media (intraframe) like Pro-res or any intermediate format so you can edit your footages. After that, you need to encode the edited footages back to a movie. The process of encoding and decoding is so much faster with a T2 chip. In fact, handbrake uses the T2 under the videotoolbox to help with faster encoding and decoding. In the past, you would need a lot of CPU cores to help with CPU encoding times, but with the T2 chip, this process is no longer necessary nor it is preferrable. So therefore, the difference between the i3 and i5 isn't much. What would make a big difference is more memory (8Gb is minimum for HD) and a set of fast USB 3 drives or Thunderbolt drives. That will make editing HD much smoother. The onboard graphics is fine for HD editing, but if you are heavily in color grading and effects, then you would need to invest in an external eGPU to help out.
To give you an idea of speed; my Mac Pro 8 core with RX-580 is about 5% faster in multi-core speed as the Core i3 on the Mini and because I also have a RX-580 GPU and 24Gb of ram and fast RAID HD and SSDs, editing HD and 4K proxy is smooth and doable even with my Mac Pro. I can't see why not with the Mini 2018 Core i3. Initially, I wanted to get a Core i3, but I saw a used Mac Pro deal and for the same price I would pay for a stock Core i3 Mini with only 8Gb ram and 128Gb SSD, I got a Mac Pro with a RX580 GPU, 24Gb ram, 120Gb SSD RAID, 2TB HD RAID and 500GB SSD USB 3. The only slowdown is the encoding part back to/from h.264. With HD, I networked my Macbook Air to help encode back to h.264 since it has Quicksync and it helped do this 4x faster than it would on the Mac Pro. My Mac Pro does not have Quicksync since it has XEON processors. With 4K, my Mac Pro does the encoding with the help of the GPU. It would be nice to have the T2 chip to help out and of course, I can add to it by buying a refurb Mini 2018 core i3 just for the purpose of encoding the 4K movies back into h.264/h265.
Movie editing relies heavily on a GPU with lots of video ram, more onboard computer ram and faster media drives and of course Quicksync and the T2 for encoding and decoding. Not so much with more cores. 4 cores is quite adequate for HD and 4K if you also have GPU support. What will help is when you are doing a lot of photoshop editing of large megapixel images and layering them, where more CPU cores will help speed things up.
Hope this helps.
