I have both a 2013 Mac Pro (6 core / D500 / 64GB / 2TB SSD) and a 2018 Mac Mini (i5 / 64GB / 512GB SSD). I have owned the 2013 Mac Pro since 2018 (it just came off of AppleCare warranty in April 2021) and have not had any issues with it.
They both perform similarly. I have a RX580 eGPU on the Mac Mini which helps with the fact that the internal graphics are pretty weak.
I also believe that the 2018 Mini will be supported for a longer period than the 2013 Mac Pro. I am not sure if macOS 12 will be the last update for the 2013 Mac Pro, or if we will see more in the future. I am hoping it will get new OS updates for a few more years.
I think the big factors in making the decision on whether or not to get the 2013 Mac Pro vs Mini, is if you need graphics support above the intel graphics on the Mini, and if you would like to have the capability of having a large SSD internally. For the Mac Pro, I am using a Sintech adapter with a 2TB SX8200 Pro drive.
The D500 graphics is not great - but it is much better than what is included with the Mini. There were issues with earlier graphics cards in the 2013 Mac Pros, so if you can find a more recently manufactured one, it will include any fixes that Apple made. I have also heard of problems with the D300 graphics and from my research the D500 seems to have the least amount of issues.
For the Mac Pro, this is a good site with lots of links in regards to upgrades that can be performed on the 2013 Mac Pro:
A guide with every conceivable Mac Pro 2013 upgrade, from SSD hacks, CPU upgrades, to eGPUs.
blog.greggant.com
Power consumption is really low. Right now I have the 2013 Mac Pro, a 4 bay OWC Thunderbolt enclosure (with fan), USB 10 port hub, Creative Soundlabs small speaker, and a Arlo camera hub plugged into a UPS and I am drawing 150 watts.
I know the 2013 Mac Pro was a polarizing Mac. I happen to really love it and think it is a really unique and interesting computer from Apple, along the lines of the also controversial G4 Cube that I always loved as well.