I play both. I love the Horn. One word about the differences between the Horn and trumpet. Most of the above posts have already said it, but I just want to add this and that is that there is a tendency of trumpet players to pick up a Horn and impress themselves with what a tremendous range they can play in the horn. That is usually delusional. The upper range is there but what said trumpeters don't have a clue to is how to tame that range as real playable notes, accurate and with pure tone. Just avoid that pitfall.
Regarding reading music and which horn to play, e.g. Bb, Eb or double horn, if you are planning on using the Horn only in a duet situation and reading Horn in F is not a given, then I see no reason why you can't simply get a Bb Horn. This means that you can play trumpet duets at will. It also means that you can save money by purchasing only a single Horn vs a more expensive double Horn. Or . . . you can purchase a better quality Horn (the Bb) for the same money than you could get by buying a double horn for the same money.
FWIW in the US most kids with single horns start on Eb horns and that has a lot to do with how Americans read and use Eb and Bb sides when they get their double horns. From that one might assume that that is the preferred instrument for a single Horn. However, in Germany, they start on Bb horns, because the Bb Horn takes precedence when playing the double horn, so if you go with a single Bb, don't let anyone try to convince you that that is an unorthodox thing to be doing and disuade you from doing it. It is perfectly valid.
The one thing that would hold be back me from getting a Bb Horn to play Bb duets with, would be that you are limiting the range of the Horn. Obviously, if you are playing duets written for 2 trumpets, you are only going to be playing down to an E Concert. The Horn can go much lower than that, and well-written duets for Trumpet and Horn would take that lower register of the Horn into account and, IMO, make the music more interesting. On the other hand, I'm not aware of a lot of duet literature for Horn and Trumpet (and piano) so you would need to research that.
As far as playing the Horn, maybe I'm being redundant, but most of the above posts have said it. The embouchure must be exceptionally sensitive, much of your playing range is at the upper partials of overtone series, so the notes are "closer" together and it's easy to make any playing a "clam bake". You've got to have accuracy and T-O-N-E. But it's a wonderful instrument and well worth the effort.