Neilsen's problems arent really getting caught offensively. Its more of bad pinches which makes his team get put in a tough spot. Than of course he gets walked on the outside so easily. He makes pretty blatant turnovers and the goalie has to bail him out. Keefe dropped him to the 3rd pairing and those are the reasons why. I think Keefe even benched him in a game just last week.
Dermott is easily better in many categories. Skating, IQ, reads, defence, managing the puck. His shot isnt as good but its actually not bad. There is a reason Keefe relys on him as one of his main 4 guys on defence. He plays top pairing/2nd pairing on the team. Keefe roles those pairings evenly. Dermott doesnt make many mistakes and when he does, its either lack of experience or its a rare brain camp.
Dermott is better than Neilsen right now and its not close at the moment really. I like Neilsen and feel he has top pairing potential but just am being realistic here. That size and shot combo can be pretty awesome in the future. If there are two coaches that can help him, its Keefe and Babcock.