Personally, I see the 'intelligence' argument as kinda desperate.
There's no real substance with it and it automatically severely underestimates Nagato's (and anyone against Itachi) intelligence and grossly exaggerates Itachi's intelligence.
Lets look at the situation.
Kabuto lost Itachi, he was shocked and commended Itachi; he did not care that he lost Itachi. He still felt that with Nagato* he could still turn the tables.
He landed a massive hit on Itachi**, B and Naruto. Thereafter he got the Jinchuriki cornered.
Itachi got a hit because he never had to actually face Nagato - this automatically tells us that Itachi could not do much to Nagato. This is why he had to attack Nagato without fighting him. Kabuto tries a CT which was shown to be an attack Itachi couldn't do a thing about***. Kabuto soon after laments about losing Nagato only blaming Nagato's immobility for his loss; in other words in Kabuto's mind, Nagato using nothing but the Six Paths (while being mobile) could have won the battle.
*Despite being handicapped. Despite Kabuto not knowing about Nagato's powers outside the Six Paths, such as sensing.
**Muu showed us that Edo Tensei take longer to recover after being hit by insanely powerful attacks.
***We've seen the damage each attack {Rasenshuriken, Bijuu-Dama and Yasaka's Magatama} can do... there is nothing to suggest that Itachi could've done a darn thing.
Nothing about the battle so much as hints Itachi>Nagato. Quite the opposite, actually.
Portrayal: Nagato was called invincible by Tobi; the strongest man in Akatsuki. Zetsu found it unfathomable that Nagato could lose, even in such a weakened state. Kabuto and Tobi didn't feel much when they lost Itachi's power, yet felt annoyed when they lost Nagato's.
Tsunade stated that Nagato's power, as the Akatsuki's leader, is what kept Akatsuki in line. There's a reason Itachi didn't go on a murder spree within the Akatsuki, despite knowing their threat level, esp. towards Konoha.
Now if you want portrayal on their power and battle capabilities: Itachi gets the general top tier hype, how he's calm, calculating and has a lot of power which can make him appear invincible and how he has the strongest physical attack (Amaterasu) and one of the strongest Genjutsu to boot.
Impressive.
However Nagato gets things like: without understanding his power, you'll never beat him. His powers control life and death (Outer Path). He has enough power to destroy, and overtake, villages himself. All inclusive to the general top tier hype.
If you want to compare someone whose met 'em both:
- Jiraiya. Saw Itachi and Kisame and thought he'd try to take 'em. Saw Nagato, he decides to go serious from the start, and this is based on Nagato's Ninjutsu skill as a child.
- Naruto. Saw Itachi, and tells B that Itachi has two jutsu which are game enders. Tells B that Nagato's powers are out of this world, right after he said something along the lines of "of course he's super powerful!!"
- Kabuto and Tobi: they generally think more highly of Nagato's power.
- Zetsu: despite Itachi's intelligence and what not, he was shocked Itachi died. Not at the concept, but at how he seemed off (not dodging the shuriken and coughing). Whereas with Nagato he was shocked that there was a chance Nagato could lose.
Interestingly, he felt it was a given Itachi would lose with disease, yet was extremely shocked that Nagato could actually lose while being in the state he was in.
The author has Naruto and Sasuke progress as a pair. Naruto surpassed Minato to fight Pain, just as Sasuke surpassed Itachi to avenge him and Konoha - by killing Danzo.
That automatically nudges us to believe that Nagato's level>Itachi's level. Alongside Tobi telling us that Sasuke should one day exceed Nagato; this means a lot as this was said when Sasuke exceeded Itachi.
When you consider all this, the "intelligence" argument (which is vague and not really strong) doesn't seem to quite cut it.