Another issue I'm sure you've read is that grass fed cattle are even worse for the environment than grain or corn fed cattle because they emit even more methane. In some studies, they suggest that grass fed cattle emit 50% more methane. The reason, according to Nathan Pelletier of Dalhousie University in Halifax, Nova Scotia, is that "it’s related to the much higher volumes of feed throughput and associated methane and nitrous-oxide emissions." Additionally, most pastures were highly managed, and subject to "periodic renovations and also fertilization." Pelletier also added that, with grass-fed cattle, "there is also a high [grass] trampling rate. So the actual land area that you need to maintain magnifies that difference". 
Sounds like a pretty stinky problem for proponents of grass fed beef. Well, Mother Earth News recently published an article on grass fed beef and stated that:
There are studies to suggest grain produces less methane, but those studies, by and large, compare conventional pastures with feedlots. However, conventional pastures contain high-fiber, low-quality forage, which produces more methane. On the other hand, studies of rotational grazing have shown decreases of as much as 45 percent in methane production, when compared with conventional pastures. All studies seem to agree cows produce less methane when nutrition is best, and the very reason for rotational grazing is to improve forage quality.
So what's a concerned consumer to do? If you are going to eat beef, make sure that you choose meat from cows that are raised in a sustainable manner. Get to know your producer and find out how the cattle is raised, if it is grass raised or if it is just grass finished. Finally, make sure that the grass fed beef you choose is from a farm that practices rotational grazing. But, really, when it comes down to it, what's the best thing to do? Eat less beef or none at all.