What makes Tsukihime better than FSN is the atsmosphere, you can feel the danger , the drama, the mindfuck.The music does a better job to add to the atmosphere than FSN.The characters interactions are so superior to the character interactions of FSN that it hurts,and the slice of life segments and Shirou's Pov are fucking annoying.FSN plot is too forced at times, some times they lead to plot holes,weak plot or unexplainable developments.Tsukihime's plot flows better, it's simple and credible..
Tsukihime's plot is also more mature, each resolution have more weight to them 1) Arc and Shiki didn't end up in good terms in Ciel's True End, 2) After every single of the Far Side of the Moon routes, Arc and Ciel will keep hunting Roa, 3) Akiha ends up as a doll in one ending, 4) Akiha and Kohaku die in another end, 5) Hisui's Good End wasn't precisely happy. In F/sn, only 2 characters couldn't be saved: 1) Ilya in UBW, 2) Saber and Ilya in HF.
Shirou never actually shows his resolve when it comes to deal with people who are prepared to kill,and the story does not have serious and mature Masters - aside from Zōken-and the only time Shirou had to confront one of his friends, was in HF against Black Saber. Shiki, on the other hand, had to fight Arc, Ciel, Akiha, and Satsuki at some point - there's also the point when he realises all what Kohaku has been doing.
FSN is just too light and aimed at teenagers. That why the dark and cold as ice Tsukihime is superior.