Depends if you're talking single or double horn?? I started on single, and was forced to double in high school, its a little strange actually using your thumb for a key, but once you're used to it, its pretty easy. I had an easier time learning FH than mello and trumpet.
I started as a french horn player in 7th grade, freshman year switched to trumpet for marching band (i wouldve been the only mello so the band director decided to throw me on a trumpet). Ive been playing mello again since February, 2 days ago I picked up my trumpet for the heck of it. I couldnt hit a single note. Nothing! So i decided to try my mello mouthpiece on it... still nothing! I dont know whats up. But since Corps is my only musical involvment now, i dont really need my trumpet skills. ah well. Its not that big of a difference 'cept the mello mouthpiece is a little deeper. Id say aslong as you keep up with both you should be fine.