Well, I'm not sure where either of you is coming from, distinguishing two men who are in fact THE SAME AGE, both as characters and real-life people. Ennis, a father figure? Au contraire! Incidentally, Jeff, if I had a cowboy lover of Ennis' age, I'm sure there would be some unpleasant term for it, too. (Bring it on, though! I can handle a nasty word or two!)
But I have another idea for an interesting way to analyze this data. Those of us who prefer one cowboy over the other may consider it a combination of physical attraction and sympathy for the character. People who like Jack adore both his sweet, hopeful spirit AND his big blue eyes and sexy physique. People who like Ennis are attracted to his complexity and vulnerability, as well as to his drop-dead gorgeousness.
So what if we imagine a parallel universe in which Heath and Jake switched roles? I don't think many of us would want it that way. But for the sake of argument, let's imagine that they are both just as good in the opposite roles, and that the movie is just as good. So the question is, which cowboy would you prefer then? If you stick with the same one, you're probably drawn more to character. If you'd switch, it may be more physical.
For me, it would be very, very, very close. But I think ultimately I would switch preferences -- going for Heath as Jack rather than Jake as Ennis. Does that make me shallow?