Diameter or Color? What's really matters?

[bdn06]

I don't like high visibility lines much. But I also have not done well in the past on heavier lines. A buddy of mine who used to catch ginormous browns on 2 and 4# test, had a theory that the fish are able to not only see the heavier lines, but also to sense the movement in them much easier. He used to fish right next to us, and anecdotally, his theory seemed quite correct. I personally think it's a combination of both.
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