Appreciate the information. If you have "the need for speed", then an SSD upgrade would be more than beneficial.
I guess an upgrade all the way to El Capitan would be OK. It's difficult for me to recommend one way or another, as I have always upgraded "on schedule and on time", ie, whenever a new OS is released (although I do not upgrade immediately, but preferring to wait for that first upgrade to the OS). Any of the 3 Mac OS versions that you could consider are stable. Again, though, take an inventory of any third party apps that you have. In fact, if you list them here, "we" can probably help you with determining whether you need upgrades or not. For some products (like the excellent freeware program Onyx), there is a specific version for each Mac OS. A prior version of Onyx will not run with a later Mac OS.
As for the need for backups, while local files might be OK, it is still wise (and just about mandatory to make frequent backups. By the way, what software are you using for that?
Finally, it would be best if you do a clean, fresh, "virgin" installation for whatever Mac OS you choose to get to. And for that, a backup is mandatory.