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With a fan shaped knot, you will have more "surface" to work with compared to a similar sized bulb knot, which makes it feel bigger than its actual knot size. Practically, I have not found any real difference, also because I do not take time on how long making lather takes.
However, when talking about the Thäters, the ones sold as two-band seem to be just a tad more firm (more backbone, as they say) than the ones sold as silvertip, while being equally soft. Then again, you cannot really go wrong with a Thäter.
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I have a fan shape, and it has worked well for me lathering on my face.
I hadn't spent much time thinking about the difference; thanks for asking and good luck.