Both branches give the same knowledge and most of the same rewards, this branch however rewards 3 Elixirs of Remarkable Will which isn't very valuable but has demand on the market.

Edit: You can still finish the questline, the black spirit one is a crossroad quest but it doesn't pop up like the crossroads in the story questline. The black spirit will have the quest "A Secretive Letter" which is the continuation of the questline and has you go to Tarif instead.

Be careful with this quest. When you finish talking to Aciel Renf, the black spirit will pop up asking if you wish to look at the letter. DO NOT accept his offer if you wish to continue the questline for the knowledge as accepting the black spirit quest locks the rest of the questline. Just got caught by this because I was R spamming.