SK-5 should be tougher than AUS8/AUS8A. (Carbon steel versus stainless. Assuming good heat treats, carbon steel is almost always tougher, if you're not talking about super steels. And AUS8 of course is not a super steel.) But it's a good question, I've never been able to find the Charpy values for SK-5 (or 1075/1080) and ordinary stainless like AUS8 and 440A, B, & C. Would be nice to know the objective values to compare them. For example, how much tougher is SK-5 than AUS8? My guess is probably about twice as tough, but that's just a guess. If anyone has a link to the Charpy values for these common steels, please post them here!