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The Balega Ultralight are very thin and rather smaller--especially in comparison to the other models. I've bought size Large in every sock, and they vary quite a bit. If you like thin, small socks, you may like the ultralight. Or go a size big if you want them to fit more comfortably.
On the other hand, the Blister Resist were way too thick and large. They had so much excess material that they felt horrible and my feet moved around in them. They are very hot. The material on the top of the foot is actually very thin, but the bottom makes up for it. That said, They make a comfortable sock to walk around in on my wood floors in the winter. Since I won't run in them, I give them use around the house.
The Hidden Contour and Hidden Comfort are similar. The Hidden Comfort are a tad thicker. They are both a bit thicker and larger than the Soft Tread socks, but they are comfortable and nicer for winter weather, when I tend to wear them running. If I were buying more, I'd go for the Hidden Comfort. Mine have stayed superbly comfortable longer than the Hidden Contour and still feel more padded.
I like the Soft Tread best because they are thick enough to be padded and comfortable, but I have to pull them snug on my feet so there is no room for my feet to move against the sock. Also, they are not so thick that I feel my feet get hot -- and I run in 100 degree Texas summer weather. Finally, the Soft Tread are made R and L foot specific, which I've found to be helpful for fit. The Ultralights are as well, but not the other three models, if I recall correctly. Though the Hidden Comfort came with an L on each sock, that stands for "Large", not "Left". Also, the heel tabs on a lot of these sock models, including this one, are great and do a lot to make sure the sock stays in place.
Hopefully this review helps a few folks decide what Balega sock might be best for them if they're looking to purchase. If you have questions, comment and I'll try to answer them.