One thing to keep in mind is comparing apples to apples.
Viking uses SK75. Amsteel Blue if its Samson, although every other manufacturer uses SK75 but calls it something different.
Masterpull uses I believe SK75 in their "Master-Pull basic winch rope." So if you are going to compare products, these are the 2 you should be comparing.
When you step up into Masterpulls superline and superline xd, you are getting into a more refined SK75. It may even be SK90, but Im not sure. Heat treating, stretching and setting, covers etc. To my knowledge there isn't anyone else on the market selling a product similar, at least here in the US.
From a personal/business standpoint, and these are based on my own experiences, Thor over at Viking has gone out of his way to reach out to me as a business owner. The ability for a business owner to look past themselves and help out one of their competitors speaks volumes to me. But this doesn't have anything to do with the product they put out. Its not difficult to put a winch rope together, so it usually comes down to price and customer service. I don't think you can go wrong with either. If I had to choose, I would go with Viking because I get the impression Viking has not gotten so big that the owner is above/to busy to keep in touch with their customers.