Lamb, i think lambs are usually free to graze and not lot fed, in Australia at least. The fat is always pure white though oddly..a butcher once told me that grass fed beef the fat has a yellow tinge.
If it was on the menu I'd go for mutton or lamb because as Tony said sheep usually graze freely. If those two options weren't on the menu I'd probably choose beef because here in the UK most cows are grass fed for most of the year and farmers aren't permitted to feed them hormones to increase their weight.
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