I like Pettersson a bit better as a prospect at the same age. I'd argue that contexts shrinks the difference in production between them. Both of them were good enough to be the offensive linchpin for an SHL team, but Willy had to play on a bottom feeder with almost no talent surrounding him, while Pettersson has played on a terrific team. That does make a difference.
I'd say direct comparisons favor Pettersson, but Willy has proven more. What it comes down to is what your team needs then. As a Leafs fan I'd rather have the proven transition expert and 60 point player with a clear upside still, rather than the arguably more talented but more unproven alternative.