The tx25c2 is an impressive performer, but its maximum output automatically steps down by 25% (info from selfbuilt's review) after 200s. Looking at the ANSI output reported by seflbuit, and the runtim graph, I suspect that the ultimate output of the tx25c2 will be under 700lm. The Viking Pro, if set to fully regulated, outputs 644lm for as long as the cell allows it.
The full programmability of the Armytek S-tek driver used by the Viking Pro is so versatile, the Viking Pro and Predator Pro are firm favourites of mine.
How often do you use the tx25c2 at maximum output? Personally I don't use the Viking Pro at maximum output all that often, and when I do, I love the fact that it runs at a constant output.
Especially with single cell lights, you have to be aware of the 'long runtime'-'small size'-'high output' trio where you can only have two out of three. I would always compromise over 'output' in preference to the other two. If I want excessive output levels I choose something else where the size will be bigger.
I would say, that if I had to choose between the Predator and Viking, it would be the Predator I would go with, as it is a great little thrower.
Regarding the whole, returning a product you have tried question, there are rules for 'distance selling'. It should be possible to return a mail-order purchase, but there are certain categories. The first is that you look at the product box and decide it was a mistake - you have not broken any seals or turned the item on - in this case a full refund, minus the postage is fine. If you have broken any seals, removed cellophane, and turned it on, then it is 'used' and if not faulty, would be up to an individual seller to decide if they can then sell a 'used' product. The refund in this case would be up the the seller's discretion. I am glad your seller has said they will be selling it as used as this at least gives some confidence they won't try to sell it as new. If the seller will take back a tested product with no fault and give a full refund, I would not want to buy from them.