A 1:12 twist generall is pretty well-rounded for the .308 caliber family of bullets other than the extreme long range pills. I know a guy who has had good luck out past 1K with the 208gr A-Max in his 1:12 twist Remmy, although I would call this the exception and not the norm. The 1:10 will generally favor slighly heavier bullets, especially ones used for hunting big game where hiking around with a heavier rifle (i.e. varmint) is not as common as doing so with a sporter weight rifle. That may be why the varmint contour barrels have a slightly slower twist, to support lighter bullets for varmints, but that is just a guess of mine. Our snipers have used the 1:12 twist with the 175gr SMK rather well over the years in the M40 series of rifles, so I dont consider that twist rate to be a handicap at all for any bullet you would want to hunt with.
Either twist should support the 150-175gr range bullet well with the advantage of the 180-200gr bullets going to the 1:10 twist (not always though, see above). Each rifle will shoot a different bullet slightly differently, best bet is to try them all out to see what your rifle likes.
