My issue with the double hinge is it tends to give the leg a "pinched" look, which actually is not an issue for me if the figure is female (recently swapped a pair of 25A Hawk legs onto a female body, and the result was very natural) but turns me off with male figures like Roadblock, as it renders the leg too skinny to look good. It's otherwise a very good style of hinge with a wide range of poses and seems to be a lot more stable in those poses than the more traditional knee hinges (good, deep kneeling and seated positions, and ideal for say, riding horseback.)
At the same time, I'm in love with the hidden aspect of the swivel hinge (I use a lot of Star Wars figures for my customs) but in that case, mostly for aesthetic reasons (you don't have the "pinched" look and the separation of the two parts is harder to notice), but it seems to be a bit less stable when posing than the double hinge, which rather surprised me (particularly with Star Wars figures), though the Henry Jones swivel knee seems to be an exception. BTW, that's probably the best example, by far, of how to do a swivel knee joint--I honestly didn't even know he HAD a knee joint until I moved the leg! Very well hidden, strong and stable...a thing of beauty in it's own right.

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