Obito would rip Itachi a new one. Itachi knew he would get rekt if he faced Obito, this is why he never went after him in akatsuki.
More over u can see in the itachi story arc, he shat himself and was stoned when obito killed his friend. Also during uchiha massacre he was taking orders from obito like a dog.
lmaoooooo. Itachi utterly stomps. Counter to KA? I thought so. Counter to Tsukuyomi? I thought so. Counter to Totsuka Blitz? I thought so. Counter to Finger Genjutsu which turns into reality simulation (one of the hardest things to breakout of, due to the genjutsu molding the very same environment)? I thought so.
Itachi >>> Obito. Ride the wave, or get drowned nigguhs.