I think both Narcissu and Side 2nd are equal titles as of regard. The main thing I found really interesting was the difference of Setsumi's and Himeko's answer. Side 2nd delves more to the idea of having something to grasp onto which can be seen at Himeko. Sure having God, family and stuff could sound cliche, but it's enough as a reason. Setsumi learns about that, but doesn't apply it or probably remember it to her life in Narcissu. This is more of the idea that experiencing the same thing feels so much different. But despite that, they both wish for others not to feel sorrow regarding that.
It's close to perfect for me, as a whole. I certainly fell in love with its soundtrack, and the artwork was really lovely especialy in Side 2nd. I have a bit qualms for formulaic stories though.
I'm also curious regarding -Die Dritte Welt-. From what I heard, I think it revolves around fantasy stuff. I hope someday we could read that too.