Well, I suppose you do get what you pay for. As Fluter points out, the Celtic Croft price is for lightweight wool, not 16oz, and of course it's not for any special weave tartan, just Irish clan/family names, of which there are relatively few. I suspect they must have some carefully guarded source of those tartans, but only in thin wool.

The price difference is not huge, so it comes down to how warm/cool you want the garment to be and/or general preferences. It's certainly helpful to know what you can provide, Matt.