I love both (don't that sound bi of me) but almost see the book and movie as two separate entities in my mind--almost as if they were stories about two different sets of cowboys! For example, in the book, Jack is described as short and buck-toothed. Neither seem to come off as particularly handsome. I loved imagining what they looked like in the book--kinda homely, tough boys who fall in love.
But in the movie, well, Jake sure isn't buck-toothed and short--he's HOT. And I can't object to hotties like Jake and Heath making some steam.
Some of the descriptions in the book are sublime but can't by nature be used in the movie; i.e. Ennis "...felt he could paw the white out of the moon" because he was having such a good time with Jack.
On the other hand, the screenwriters added some fabulous touches -- the "no more beans" leading to Ennis dutifully telling the grocery guy, "I'm sick of beans"--how sweet! It's an almost domestic touch showing that Ennis is trying to accommodate Jack's tastes (or shares them). Even though it means ordering soup--which Ennis "doesn't eat."
I could go on and on but won't. Geez, BBM almost makes me miss my days as an English major writing essays! (Almost).