Yeah, now they're similar, but at the time of Sasukes and Deidaras time, Itachis abilities were better, even the Mangekyou Sharingan not included. Both are using the Sharingan, but Itachi on a higher level, Itachi is faster than Sasuke, he's more experienced and way smarter, means he's able to analyse the situation immediately and create a reasonable plan.
Sasuke outran Amaterasu, but actually this was a whole fake fight where Itachi intended to reach his limit as fast as it was possible. Amaterasus effect is known, everywhere in the sight of the Mangekyou user arises in those flames, not only it's hardly possible to escape from a humans sight, the Sharingan can follow your movements. And here, Kakashi is the best example. While Amaterasu is cast instant, you have to aim at the target with Kamui until it has absorbed it, and we're talking about Kamui in its initial stage, which wasn't even fast. In plain english, if you can't really escape from the Sharingans sight, you can't avoid Amaterasu. At this point it doesn't even matter whether Amaterasu hits the bird or Deidara himself. Once Amaterasu is cast on the bird, it gets covered by it completely, whereby Deidara get in touch with it anyway.
2. Although Deidara trained his eye, he yet got caught by a Genjutsu again in the end:
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And while Sasukes Genjutsu value is 4 according to the databook, Itachi has the full score and can use a perfect Tsukuyomi, which cannot be avoided by Deidara.
Even Susanoos activation is faster than Deidaras bird he has to build first. So actually Itachi can crush Deidara with Susanoo before he have built hi bird.
Itachi has definitely the better cards here. He can even cast an illusion on Deidara while he's flying far above, since he can give his crows the Sharingan in order to cast an illusion on his opponent through them.