Don't even think of getting a dog unless you will be fine with it never riding with you, because no matter the breed, in some cases it just won't work out, and you had better be totally OK with that. And at some point ANY dog will be too old to do it.
That said, I did have a dog once that was a great trail companion, and that was a real joy. I am also fine with people bringing dogs along on rides and with dogs I encounter out on the trail.
Dogs really don't ask for much, I am fine sharing the trails with them.