Ethical considerations are a big concern with down, neither of the bags linked mention anything about traceability or accountability of the down used within. Go to any of the big name companies and they all subscribe to some kind of scheme whereby you can trace the origins of your down or they offer reassurance that the down hasn't been live plucked.
As well as the ethical concerns, I would be concerned about the quality and construction of the bags, do the bags have baffles to stop the down migrating around the bag? are the baffles overlaid to stop cold spots forming? do they have neck baffles? are the drawcords easy to operate and find when you're snuggled down inside the bag? What about long term durability? packability also is meant to be a big advantage of down, though neither of the linked bags seem to pack that small.
For budget down bags my first port of call would always be Alpkit, they have a good range of bags from two season to four season and all of their down is certified to come from ethical sources. They also have excellent after sales support. Whilst they don't have bags for 100 pounds, they prices are still pretty reasonable for the standard of bag you are gettting.
Rather than using a second bag, I use a down jacket to supplement my warmer sleeping bag, so I can keep warm in camp and then at night if the temperatures are low, I'll wear it in my sleeping bag.
