The French Horn usually gets beat out by the Mellophone for Marching. They are easier to tune and due to the thicker rimmed mouthpiece it’s less likely for shocks and bumps to cause disruptions. It is also is more distinct from the soprano (trumpet) and the Baritone, with a brighter/more piercing sound that cuts through.
It is all really about preference though, I like to keep a French Horn in for the lower register. Worth checking out both if you’re looking specifically for a Marching instrument.