Go for Nathan!
Trust me! You've actually had a past with Nathan and the two of you share experiences together. If the relationship between you two does not work out, you'll still be great friends. If you choose Dave, you'll not only enter a short relationship, but you'll also destroy any chance you had with Nathan. Nathan will definitely feel terrible that you decided to date his brother instead of him and if you ever try to go out with him later, he'll think you chose him as an alternative to Dave, and no one wants to be the back-up plan.
If Nathan does like you, which I think he does, you'll find it very rewarding that you decided to persevere.
I think you like Dave because he's cute and you're 100% certain that he is gay. You like Nathan because you actually fell in love with his personality. Even if you're not 100% sure if he is gay, you should definitely find out! Nathan seems like the type who actually cares about you. He's the one who wants to go out with you for the rest of his life, and he definitely seems devoted to you, alone. Dave, on the other hand, is just looking for a fun time. Once he's done with you, he'll quickly more on to someone else.
Nathan, is definitely the right choice