"Best place" is where you'll use them, typically at the machine there they will be in easy reach.
I have that style gate, they do have issues with sawdust collecting where the slider engages when closed so I keep that side "up". Overall they do work well for the price. $18 is wayyyy to high, I paid maybe $8 a few years ago so
shop around (or make your own!)
As to placement, one thing I found is that the DS needs access to the drum for paper changes and I like to spin it by hand as I lower the drum to just kiss the wood when setting the initial cut. It's not a big deal to pull the hose off/on, but a cleaner setup is to install a PVC 90 elbow on the cover and run a short length of flex hose to a coupler attached to the DS base. You can install the gate on the base or at the cover. This way the cover can flip open without dragging the full length hose with it.