The both are very good. Do research on the breeds. Pick a breed that has personality traits you like. For example, Do like a horse that is very high-spirited? Or maybe more relaxed? On the short side, or tall?
Discipline also depends on some breeds.
Personally, I would choose a discipline first (for me, western riding in general) from there, color, breed aren't important. I look for personality. It's best to get a horse that has a personality similar to yours. If you are lazy, get a lazy horse that doesn't have to be kept in shape every day. If you like to 'go-go-go' get a horse that always has spunk and energy =] If you can't devote that much time, get a well broke, older, easy keep horse. If you have time to devote 'how ever long it takes' you have more options there.
I caution you however, don't pick a horse until you are absolutely sure that is what you want. I made the mistake. I am in this hole right now. I saw him online, located 10 minutes away. I drove out to see him, gave it a few days, then decided to buy him. I rode him three times, did great. He is green broke, but he did so good right? WRONG. The last time I rode him, he reared on me. I was unprepared. Yes, he had a personality I liked, he was an Appy, another plus, but, I thought I could handle him, and I can't. Luckily, I only put a down payment on him, so I'm going to cancel the contract. But don't let people pressure you, like they did me. They said 'well, if you haven't decided in two days, we will sell him to the other person coming to see him'. So I made a quick desicion. The first two times I rode him, were in a round pen. The third time, the man was with me. When he acted up, I was alone. Just be cautious =]