Ah, it's a philosophical distinction in the end.
Evasion is rooted in the intent to deceive, with a willful desire to circumvent the law, irrespective of the intent of the law. The evader cares not what the law says, but only that they not be caught.

Avoidance is based upon a careful analysis of the law, always with an intent to comply. The intent is to be legal, to use the provisions of the law in a clever but compliant manner. There is no deception, no willful desire to circumvent the intent of the law.

Unfortunately, the vagaries of language and the complexities of transactions leads to a fuzzy boundary between avoidance and evasion, where the distinction sometimes turns on intent.