We swapped my wife's 500GB HDD out from her 2011 MacBook Pro for a 500GB SSD and it made a world of difference, easily extending the life of that laptop by another few years. You won't regret going SSD. It's easily the best bang for the buck upgrade you can make in computing right now.
And if I were building a computer right now, I'd go M.2 in a heartbeat, particularly for SFF, if for no other reason than to save space, cabling, and complexity. The price difference isn't too bad and the benefits are—at least to me—well worth it. As for NVMe, while I'd love to go with it, the price different is tremendous while the performance benefit is marginal for the vast majority of people. If you have a specific use case where it would make a difference, give it due consideration, but otherwise I'd think you'd be well-served by a SATA SSD, whether M.2 or not.
As for Optane, it comes at a steep premium as well. I briefly toyed with the idea of using it as a cache sitting in front of a RAID, but it wouldn't provide much benefit to me with the way I was planning to use it (media server storage). Again, if you have a specific use case that would benefit from it, give it some serious consideration, but most people aren't particularly well-served by it just yet, though within a few years I hope that will change.