Personally, between SSDs, I would say it depends on the use. If you are building gaming PC or just se omething gor everyday use and yiu aren't already on high end components, then I would recommend bigger SATA SSD or equivalently priced NVMe, like Intel 660P 1TB. Game loading wise it is close enough and everyday stuff aren't that demanding, so you will notice bigger size far more than second or two off loading times. Plus extra money can be better spent on CPU and graphic card, if it allows you to go one tier up.
If it is high end build, like you already have i9 or R9 and 2080 or 2080Ti, then by all means get best, this clearly is Cadillac build and you got money to burn, so you might as well get best.
Otherwise only real reason to prioritize good NVMe drive simply is work. If it allows you to do your work faster or smoother, it always is good investment and beside that, when it comes to work, we are talking about return on investment too.
As for regular HDDs, if you need big 3TB+ storage, they are stil worth it. But otherwise SDS got cheap enough to where 1TB SATA SSD is affordable and miles better investment. And even 2TB is not that terrible. Only reason to do it is really low end build, where it is still worth combining 256GB SSD and HDD, to cut costs as much as possible.
As for StoreMI, personally I don't like idea of losing control over where stuff are. Also it highly depends on training to load stuff faster and it depends on consistency of your habits. It does fix fundamental flaw if SSHDs, which was that SSD portion was too small and they had to do too much swapping to be useful. But still, I would prefer to have everything on SSD, so I never have penalty of something not being on SSD.
In my PC, I just have 512GB SATA SSD for OS and 1TB SATA SSD for games. For long term storage I have 4TB external USB 3.0 drive, with classic HDD inside, which I just plug in when I need it.