The MF is super water resistant. Unless you camp in bivies in the PNW you just don't need the GWS in my opinion. One look on Ebay will demonstrate that Western Mountaineering bags last a lifetime. There are 20 year old bags on there that will still hold their own in the field.
I could not disagree more with saying foreign sourced materials make the difference between made in USA vs made in China splitting hairs. WM sources materials from USA when possible, and the actual work of creating the bags is done by hard working, highly skilled Americans.
WM will always have better resale value, and I would bet $200 that you will not have buyer's remorse after spending $200 more on the WM. Just my opinion as a WM fan boy.