Theoretically, down, because that will maintain a siphon slightly longer. But I'd recommend you instead consider a braided hose that lies on the bottom of the tun, especially if you batch sparge, but maybe even if you fly sparge.
So I keep reading about those and see reference to plumbing hoses. I assume I get one and can pull the rubber hose itself out and use the metal jacket?
Yes. It's commonly calle "toilet braid." Hack off the ends and push the core out, leaving the stainless steel braid. Crimp one end of the braid, and clamp the other end around a barb leading to your tun's spigot/ball valve.
Should I try to cover a lot of surface area of the cooler bottom or just say a 16” straight piece and let er rip?
If you are fly sparging, covering a large area can reduce any tendency to channel. If you are batch sparging, there's no reason to configure it in any special way, other than to keep it against the bottom.
The braid works great. Never had a stuck sparge and I've used way too much oats and wheat in the mash a few times. I use a ~14 inch braid in 17 gallon chest coolers for batch sparging.
