Well, for Sasuke, his whole clan was destroyed along with his father and mother, and I assume him wanting to "Avenge" the death for his clan and parents was by killing his brother (Who is actually trying to help him) was kind of a fail to me xD I mean, after Sasuke found out it was Konaha's fault, he then went on to "avenge" the killer of his clan (being Itachi) by destroying Konoha now. I dunno. To me Sasuke is just really confused on the matter, so i don't really think he fully knows what "avenging" is. I did however, like his reason in the beginning of the show but since he has changed his mind over a gazillion times....it's getting kind of old now imo.
As for Obito, yes he is doing it for a girl, but we have NO idea what the full purpose this actually is for. Yes, the new chapter came out, but his side of the story is STILL going so he still may have a legit reason as to why he he wants to avenge her. The odds of this turning out to be just for her because "he loved her" would be really idiotic and I don't think Kishi would really have this whole "big strong tough bad guy" have such a lame reason like that. I think we are calling it a bit too early, but since Obito's reason isn't really done yet...I would have to say Sasuke, even though he is confused lol My opinion though.