I would advise you to visit Diamond City first
can't imagine how it should work otherwise oO
I've been to the other first. Some people there will tell you to go to diamond city and meet nick if you choose certain dialog choices.
Once in Diamond City you can bypass Piper by passing a speech check with Arturo.
I went to Diamond City because that was the next objective in the main quest for me. I am level 50 and I have yet to speak with Nick Valentine to advance the main quest. I did a few side quests, but not many. I've been focusing on gather resources to build 5 settlements. Not sure when I will pick up the main quest again...