I'd say it depends on the context. If you're discussing something which happened, and you're trying to decide what the consequences might be, you might use "practically speaking" and "legally speaking" to distinguish between what might happen if the letter of the law is followed as opposed to what might happen if things turn out the way they usually do. If you're talking about the meaning of a particular phrase or sentence, you might use "legally speaking" to interpret the sentence as lawyers would, as opposed to the way a "layman" might read it.
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