I would say her Avengers hair is most practical for when she's being a superhero but when she's being a spy, the TWS hair is better. Logistically, it is thin and somewhat short so it can be died very quickly or put under a wig easily. It's also long enough to be able to do stuff with it though. (This isn't my idea, saw it on a blog) Also, it is an obviously died red so that when people look at her they think, redhead, and don't pay as much attention to the rest of her so its easier to slip away with a small change. Same thing as Fury, even if the whole world saw him in his black trenchcoat and eyepatch, almost no one would recognize him in a hoodie and sunglasses.
Its a simple principle of disguise, wear something that attracts attention away from what you actually look like so you can disappear in other clothes easily.
I've always kind of wanted a scene where Black Widow goes into a safe room while she's on the run and we see a selection of red wigs in various different style followed by the reveal that she's bald and is always wearing a wig. (This would have the obvious impracticality for a spy of being dependent on a wig hence the "kind of".)
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