I know you're interested in high brass, but baritone would really be a better idea. It's easier to learn, and should you change your mind and decide to march, there are generally not as many baritone players as there are trumpets/mellos making it less competitive. If you're set on high brass, mellophone would probably be easier to get the right notes, but getting a good tone is really hard for a lot of people. As stated, it's difficult switching from high brass from low brass due to embouchure differences, so you might be better off starting with high brass and then experimenting with low brass if you've got the time/patience. Good luck!