They graze on grass, clover and
wildflower meadows and are fed peas,
barley and hay in the winter.
Grassfed is definitely best because it'll contain more micronutrients (vitamin K, E, CLA etc) and a better ratio of omega3:6, but it'll also be leaner and contain less SFA.
The controversy over leaner animals versus fattier animals basically comes down to Cordain and his erstwhile opposition to saturated animal fats. By going for grassfed you're getting both less fat in total, but especially less SFA. Cordain believed that our prey in our natural evolutionary environment would have been leaner and so would have provided a small amount of 'healthy' PUFA and little SFA. Conventionally-raised animals, conversely, contain excess SFA and thus might explain why (allegedly) lots of meat and animal fat is bad for us (via all that dubious SFA) but was the mainstay of our evolutionary diet.
Most people here, and (largely) Cordain more recently, think that there's no reason to avoid SFA, because the whole lipid hypothesis about SFA driving cholesterol driving CVD is full of holes. The important question is what you would replace these lost SFA calories with if you avoid them out. Cordain proposes plants and nuts. The nuts are unquestionably a bad idea, since the excess omega-6 is a recipe for disaster. The proposal of eating lean meat and tonnes of plants is more plausible, with the qualification that the plants will simply be converted into SFA by your own body. The main difference is that getting that SFA within your body by eating a tonne of carbs will entail large amounts of insulin and rising-falling blood sugar- it seems to work for the kitavans though, just as eating straight SFA seems to work for the inuit and masai.