Alright, so here's why Tobirama stomps. He has Flying Thunder God, which mortally wounded a Mangenkyo Sharingan wielder. Also, Izuna was much more used to using the Mangekyo Sharingan then Sasuke. If your wondering why I didn't include Shadow Clone Jutsu or Summoning: Impure World Reincarnation, it's because Sasuke has seen Shadow Clone Jutsu way too many times, and we haven't seen Tobirama using Summoning: Impure World Reincarnation. Also, I'm pretty sure Tobirama has a moral compass. Flying Thunder God was literally made to counter the Sharingan's enhancing of reactions. The Sharingan can't detect stuff that's instant.
