I think pier25's point is we should spend our time advocating for the most important issues. See the forest, dont focus on one tree. It is true that beef has a large impact, but maybe time would be better spent advocating renewable energy subsidies and or carbon taxes? You can only complain so much before people get sick of you right? :P However I think their point was weakened by the anecdotes about vegetarians who do other unsustainable things and calling them delusional.
I think cryodesign's point is that every little bit counts. The forest is made up of trees after all. And it easier to avoid meat than it is to invent renewable power that can provide base load.(well I love meat, but I have been reducing my beef consumption) I think they focused too much on how it feels. You can humanely kill an animal imo. As someone who grew up on a farm, I found the quote about killing them with a pipe unbelievable.