The problem is we only saw Itachi use Susanoo once. So really we have no idea whether he was able to partially manifest it or not. In order to defend against kirin he needed a full Susanoo with the Yata mirror, so he created a full Susanoo. Its very possible he could do the same as Sasuke has been doing. Also we don't really know if Sasuke was purposefully doing partial manifestations, it may have been that he just couldn't summon the full Susanoo yet and those things were the most he could do. Plus Itachi's had the Yata mirror and the Totsuka sword.
But I will give you that Sasuke is more talented with Amaterasu. Itachi was never shown to be able to use it as Enton release, and also he wasn't able to manipulate the flames like Sasuke can.
But as far as Tsukiyomi and Susanoo go, Itachi was more talented so far.