Well, there's one thing that should be pointed out. That is -Ennis was a responsible person, and Jack was easygoing. Ennis would persist and endure anything if he felt that it was his obligation, but Jack... uh, funny and dear Jack! He would just leave the sheep to separated themselves,
! So, maybe Ennis' words were his way of persuading Jack to do what was necessary. To explain Jack the reason why they should do it. If you need this job next year, than this year you just have to do it right!... Don't know, but could be... Cos mentioning next year for Ennis was not a new summer love with Jack. I think that even then he knew which path he's going to chose. Poor Ennis! :\'(
I've just read about the cowboy who was Heath's inspiration for Ennis' character - that is an amazing story! That cowboy had passed very similar inner combats as Ennis, only he ended much happier. Thank God! 
I’m going to jump in here in defense of Jack. I think it’s interesting that so many people have that perception because I don’t see them that way.
In this particular scene, I’ve always liked seeing the way they complement each other.
Ennis, the new hand, turns to Jack, who had been there the previous summer and asks: “Well what are we supposed to do now, huh?”
Jack says with both authority and casual resignation: “Get on in there and untangle them Chilean sheep out of ours, I guess.”
Very matter of fact, that’s just what has to be done. Then while they’re working at it, Jack is griping, in typical Jack fashion, about the paint brands, etc and Ennis now shores
him up by reminding him they need to hold firm and get the job done.
When Jack says “f*** Aguirre” I don’t think he means to indicate he’s not going to finish the job, just that he isn’t doing it for Aguirre. He’s doing it because it’s part of the job. Just as complaining about the paint brands doesn’t mean he’s going to quit; he just likes to complain.
I thought this was a great depiction of the way they seemed to work so well together throughout the summer. They just seemed to fit, much like those lock tumblers AP refers to in the reunion scene in the SS.
As to their work ethics in general, there’s really nothing to indicate that Ennis has any stronger work ethic than Jack - he just shoulders his responsibilities quietly where Jack likes to complain about his. In the SS after they get together in FNIT, it says
…“without saying anything about it both knew how it would go for the rest of the summer, sheep be damned.” I think that both put as much into the job as any 19 year old of the time, doing what needed to be done and at the same time willing to let things slide occasionally to spend time together.
As I said, I really like this scene for a few reasons, one of them being the beautiful balance I see between the characters and the way they seem to complement each other so well. I also love Jack’s “f*** Aguirre”

and then the look that Ennis gives Jack as they return to camp, right after his comment about the harmonica.
