I've had both, but not both at the same time.
I'd say it depends where you got your fish from. I received an achilles tang direct from a diver in Hawaii and had no issues at all with the fish. He came in with ich but was easily treated and since my display was treated he never contracted it again.
I've received two powder blues. First one was from a LFS, not sure where they got their fish. It wasnt healthy when I bought it. This was when I was first starting out in the hobby and didnt know much. I put it into my reef, where it started eating well but continued to have ich problems. I never treated and it eventually died, although it hung on for a couple months before kicking the bucket. All other fish in the reef did OK, it seemed to be the fragile one which is pretty much what you hear from other hobbyists these days.
Second one I got from Live Aquaria, one of their Marshall Islands powder blues and it was so much healthier than the one I got years ago from the LFS. No ich, never showed signs of ich and after a month in QT I put it into my tank and never had a problem with it. It eats great, doesnt cause problems with the other fish and was super healthy as far as I'm concerned.
Anyway, I think either fish can be hardy, depending on how good of condition it is in when you get it. I'd give the nod towards achilles as being the more finicky of the fish, but in my experience once they are doing well in your tank, they are not hard to keep. It's just getting your tank totally disease free and low stress that is the hard part to keeping an achilles
Just to show he wasnt the only tang in the tank, he had some larger buddies but handled it totally fine.
And here is my totally disease free (as far as I can discern) powder blue.
Again, I feel the key to keeping both of these fish that are labelled as hard is keeping a disease free aquarium. I wouldnt plan on keeping either if you cant spend the time/effort quarantining all your fish.