I believe I've answered my own question. I've been testing back and forth between my Rift and my 50" 4k TV. It's MUCH easier to spot other aircraft on the TV when the resolution is set to 1920x1080 verus 3840x2160, even when the terrain is in the background. Using a low pixel density helps with the Rift, but not nearly as much as using a low resolution on my TV.