This is a tough question. Karlsson doesn't have the track record and this was his only season as a top 6 player where as in CBJ he was bottom 6 defensive C. Having said that this year, regular season and PO's considered, by far trumps anything Nylander has done. The guy is a tenacious defensive player while actually playing as a top line C, and has been 0.5 GPG/0.94PPG for nearly 100 games this year.
Obviously given the question, there is tremendous risk that this is an aberration, however the innate chemistry he, Marchessault and Smith have wont allow his production to dip below ~30g ~30a/60ish pts player IMO. I am going to go with Karlsson, reluctantly.