I learned the mellophone for marching band this year(I'm a bass clarinet player... long story.) Anywho, I'm thinking about asking my director if there are any spare horns I could borrow after marching band is over to learn to play. What are the differences between playing mello and horn? I know the fingerings are different, but what effect does the cup vs conical mouthpiece have on embocure differences? Would having learned mello make any difference on how hard it would be to learn horn?

I play mellphone with my french horn mouthpiece and an adapter. I'm not sure if that what you are doing or not. If you're not then you're probably using a trumpet mouthpiece.

French horn sounds better, and is a lot easier to keep in tune. Also, you have to use a french horn mouthpiece on a French horn. Yes, fingerings are different. Mellophone seems to be easier to play high notes, but the range on French horn is nearly twice of mellophone if you can play low notes.

i'm gonna learn mellophone for marching season, it's the only way to stay out of the pit... but i look forward to it too, i suppose... a horn i can hold like a trumpet, figures. anywho, i suggest you learn to play a horn. it's a real joy to play, i tell you what.

that mellophone wouldnt be so hard to tune if you didnt uset the mouthpeice adapter on it.

knowing mellophone helps with pitches on the french horn

I've been playing the French Horn since 4th grade(I'm a junior now) and just took up mellophone for marching season. I don't use an adapter for my french horn mouth piece because I don't use it. I find it easier to play the mellophone, but the french horn definitely sounds better. It's easier for me to keep my mello in tune then my french horn, but, it might be because I just started using the new one that came in next week. Iunno. I like them both the same. Eh. I don't even know why I posted this xD.

I use a 3C trumpet mouthpiece on my mellophone, because that's what was in the case when i got it from school.

I've only played mellophones occationally, but I find that high notes are just as easy on the Bb side if you have a double horn as on mellophone. The difference in fingerings is just from C#-E, which is 4 notes. Plus, horn definately sounds better. Mellophone sounds like... a trumpet... with a more hornlike sound... or something. Whereas horn is that hornlike sound. It makes more sense if you just hear it. As for a mouthpeice change, you will be fine after playing it with the horn for a couple days. Don't play mellophone while you're first learning horn, though. I tried to play trombone while learning trumpet... didn't work.
Brad