and playing against teens doesnt advance development against nhlers.
His development continues, regardless of where he is playing.
You keep him in the NHL and he is working hard on his defensive game and positioning, trying not to be a liability while he is on the ice so that the coach will have faith in him and give him more playing time. This route he will likely be a better defensive forward sooner, but he is literally getting 20 seconds of puck possession time in his 8-9 minutes per game.
In Juniors he will likely be getting several minutes of puck possession time per game, and learning to be the go to guy under pressure, while likely pushing his offensive creativity to new heights. His focus on defensive positioning will probably not develop as early as the NHL version's would, but there is a better than average chance that his offensive skillset would be better developed in lesser leagues.
So what matters most to you? Having an elite scoring center? Or a solid two-way center with more defensive chops, but not game breaking offensive ability?
Development routes absolutely influence different areas of game development. There is no guarantee that all aspects won't eventually develop and hit their ceilings, but there are greater chances of prospects being better in certain aspects of their skillset depending on where they develop.