Itachi all day... Itachi was sick n weak, neither tobi or itachi were scare , plus itachis plan was never to fight tobi, it was to die by his brother n pass on his powers,..
To be honest, "mutual understanding" wasn't just needed between Tobi & Itachi, it was all needed between the leader and the rest of the Akatsuki members as a whole to be more effective & dangerous. As far as one can say, Tobi created that organization to make use of it or to better aid him completing his goals and not obviously the other way around.
Anyways on topic, Tobi wasn't or will never be afraid to fight Itachi or any other member of Akatsuki. This has been shown as he was so confident to take on Kabuto even with the presence of the 5 strongest members of Akatsuki in front of him. This of course includes Edo-Itachi, Edo-Nagato, blah.. blah.. & so forth.. The fact that Tobi decided not to pursue on invading Konoha when Itachi was stil alive, doesn't necessarily mean or prove anything that he was intimidated by Itachi's power. There were reasons why he intends not to attack the village because:
1.) It was part of their arrangements, or as long as Itachi performs his tasks as an Akatsuki, their agreements were still honored.
2.) He wanted Itachi to die at Sasuke’s hand so he could manipulate Sasuke on his own way.
3.) Itachi joined Akatsuki in order to keep an eye on the organization. Thus, it could deteriorate most of Tobi's plan because Itachi could provide ample intel's to the rest of the villages about the Akatsuki's doings, goals & purposes, strategy, member's strengths & weaknesses or their jutsu's & capabilities.
Furthermore, Tobi seemed to be one-step ahead against Itachi. He knew the truth about him, the reasons why he wanted to die at Sasuke's hand, his real purpose of joining Akatsuki, & fooled him by letting him believed he was Uchiha Madara. Also, Itachi's plan for Sasuke failed only because of Tobi's intervention.
Hitherto, it was never stated in the manga that Tobi wished to kill or fight Itachi. However, one of Itachi's main goals is to kill Tobi. He even planted amaterasu in to Sasuke's eyes hoping it will eventually kill him.
So my conclusion, pre-rinnegan Tobi > ms Itachi. Itachi even believed that Tobi (a.k.a. Uchiha Madara during that time) was invincible & immortal. If he could actually kill him, he must have accomplished that a long time ago.
OT: People underestimates Tobi’s S/T often times. To every battle we’ve seen him fighting, he never had the intention to kill. His plan is to capture or warp them in to his dimension & perhaps gain control of them through genjutsu as we witnessed what happened to Fu & Torune. Even when he fought Konan, he could not kill him first because he wanted to know Nagato’s whereabouts. Imagine what will happen if he goes for instant kill, he could aim for their heart. Thus, making his S/T a “one-shot jutsu”!
