Both can have their place depending on the area but I would save your money and hunt first before you buy anything. You'll use 8x or 10x binos way more and if you don't have a good set in this power I would invest there and on a quality tripod setup even if just for the 8x or 10x's.
They are 2 different tools. Each have their own use. If you're new to huntin the west try a good pair of 8 or 10x like dot man said and glass off a tripod. If down the road you find that you're hunting big country and love glassing for hours then go for some 15x. They are an amazing tool but don't fit everybody's hunting style. If you do get some I'd skip the Kaibabs and look for the older minox 15x58. Less money and better glass.