An internal SSD will be slightly faster, but you probably won't notice for day to day usage. I booted my 2012 quad-core Mini from a 1TB Samsung T3 USB SSD for a few years and used demanding software like Final Cut Pro and Logic Pro. Worked just fine for me. The only noticeable difference is that it takes the Mini much longer to boot from the external SSD.
Mine has an original internal 256gb Apple SSD and could boot into the desktop in around 15 seconds. The external SSD took more than 30 seconds. But after that, I don't think you could tell the difference. You have to do a lot of disassembly to put a new SSD inside the Mini. Personally, I wouldn't do that,there are chances of damaging something, especially on such an old computer.
This was a test of the original internal Samsung sm256e SSD on my 2012 Mini
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And this is the external
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