Outdoor painters work on many different surfaces. I know one painter who even works on copper! But for me, it's usually a choice between canvas or hardboard panel.
I prefer hardboard panels. The reason is that, unlike canvas, they're puncture-proof. (The area around Sedona is thick with thorny vegetation like catclaw acacia, mesquite and prickly pear cactus.) I can also get more of them into a wet painting carrier. If I want a store-bought panel, I use Ampersand Gessobord or Claybord. If I want some interesting texture, I'll make my own on untempered hardboard sealed with Gamblin PVA size followed by a coat or two of Golden Acrylic Gesso randomly applied. I don't use linen or canvas on boards because I don't like to paint on a woven surface placed against a rigid substrate.
Unfortunately, above a certain size, hardboard panels become heavy to lug around. My solution for this is to use stretched canvas, which will be lighter than any hardboard panel over 16x20.
But canvas has its own problems. First, it's not puncture-proof. Second, the canvas transmits light. Anyone who's painted on canvas in a brightly-lit desert will have seen the effect of light passing through the canvas and illuminating the painting from behind. It's nearly impossible to judge value and color that way.
My solution for this is to insert a sheet of foamboard, cut to fit tightly within the stretcher bars, and to secure it with framer's points. If you use something like a gun (Fletcher's FrameMaster) to shoot in the points, you must be careful to not shoot the points through the canvas! I prevent this by gently pushing up on the canvas from below, which raises the board up slightly, letting the gun shoot the points a little higher up on the stretcher bar. See the photo above for how this would look when done.
By the way, in case you missed the announcement the first time, I have a free 7-minute pastel painting video demonstration on my YouTube channel.