I love the grass fed stuff. We buy 50 lbs of it twice a year and it's pretty much the only beef we eat, unless dining out somewhere. I find that GFB has a completely different flavor and texture. Part of that comes from the fact that the animals are much older when slaughtered compared to grain fed. The meat is leaner and stronger flavored. Because it's leaner, it can also be easier to overcook.
Now getting back to the original topic of portions, below is the portion size of steak I normally eat - in fact this was tonight's dinner. This is 4 ounces of grass-fed tenderloin. DW and I usually split an 8-ounce cut. The American Heart Association actually recommends a 2-3 ounce portion, so 4 ounces is pigging out a little. Yeah, I realize it probably looks like a mere nibble compared to the 72-ounce steak mentioned earlier in the thread, but I just cut it into a lot of very small pieces and savor each morsel.