Depends on what game and use-case you have for SLI, I use 2 GTX 960 4GB Cards in SLI (and I know i'm going to get memed for it) but I mainly use it because I couldn't buy a 1080 TI at the time (1800+ USD, thanks miners) so I popped my 2ed GTX 960 I had lying around and started using SLI again. Though if you are going to use SLI, NVIDIA Inspector is a MUST as you can force a game to use SLI, and has other benefits like Multi Display Power Saver.
Dualshock 4. Works wired, works over bluetooth, has the best D-pad, and by installing Input Mapper you also get the touchpad (which can actually be pretty handy), Xinput support, full rebinding and automatic profiles. I'd dare say there's no better pad for PC.
The Steam Controller, imo, is some kind of middle of the road freakazoid: it enables you to play games meant for keyboard/mouse on a sort of pad, but it's very much inferior, and it is also inferior to a traditional pad in games that are optimized for them; say Dark Souls, for instance, or a fighting game, or tight platforming games. I like the idea of dual action triggers, though, but yeah... it's pretty much a jack of all trades, master of none. Also, try to press the X button without putting your hand in a very awkward position (unless you're a giant); it's like pressing start on the Dreamcast.