1. Versatility: Currently SX OS, based on XCI support and Emunand. That being said, Emunand is coming soon™ to Atmosphere, and there have been arguments for a long time about whether XCI loading makes SX OS better than Atmosphere or whether it's a feature worth paying for.
2. Stable: All CFWs are pretty stable as far as I'm concerned. The important thing is to use a microSD formatted as FAT32 rather than exFAT, especially if you plan on using homebrew that writes to the SD card, like emulators.
3. Brick Risk: I don't think this is much of a factor. Just make sure to keep a good NAND backup. There are also instructions available for making a "fresh" NAND dump from scratch (which could easily get you banned if you use online, but it's better than having a bricked switch if you need to resort to it.) At this point, if a NAND restore can't fix it, it's probably a hardware problem.
4. Possibility of Tools: ReiNX and Kosmos both have tool kits, plus there are plenty of homebrew "tools" available that work on multiple CFWs. SX OS has ripped off and "re-branded" various homebrew tools as well.
It's mostly a matter of preference. If you want to be able to run .XCI files (which means being able to run cartridge dumps directly, without installing them first), or you want to use emunand and don't feel like waiting for Atmosphere to release it, then pay the extra money for SX OS. Otherwise, Atmosphere, ReiNX, and Kosmos (which uses Atmosphere) are all functionally about the same, with relatively minor differences between them.
If you like manually setting everything up and want to feel in control over everything on your Switch (like me) use Atmosphere, and then you can set up all the sysmodules / homebrew / patches you want.
If you're not interested in that and just want an easy setup, use Kosmos. It comes preconfigured with sysmodules for cheats, a background FTP server, overclocking (if you want it), Amiibo emulation, and more. You will have to manually install sigpatches if you want to load backups, but that's an easy thing to do.
Also there's a very handy chart you may want to look at here:
https://guide.sdsetup.com/comparingcfws