Could go either way honestly. But Itachi takes it high-diff. Maybe even med-diff if he's lucky.
Saying Tobirama wins because he's fought MS users before is truly reaching. Primarily because we don't know the techs of those MS users and how good they were in genjutsu. Itachi's genjutsu looks entirely normal until he decides to distort things and allows you to know that you're in it. He's also the only Uchiha we've seen to employ non-ocular genjutsu. It's not a matter of Tobirama breaking Itachi's 3-tomoe genjutsu, it's a matter of realizing he's in it (which he may or may not- some of his sensory abilities have been inconsistent with the way Kishimoto wrote the war arc) and having enough time to react. What would happen if he was caught in a 3-tomoe genjutsu, then Itachi quickly made him look into his eyes so that he could use Tsukiyomi to end it? It would be over. Itachi also has a varied arsenal as well. Not an OP one (though he does have a few OP techs), but a decent one nonetheless paired with great intellect (which Tobirama has as well). Tobirama's is much more simplified, primarily consisting of FTG, shunshin, clone feints, some pressurized suiton, and things of that nature. He'd need more to take Itachi down.
Also, another thing to consider is that saying he "soloed Izuna" isn't very legitimate. Izuna had been his rival since he was around 10 years of age... he didn't manage to kill him until they were nearing their 40s. :| They were also in a group with nearly every Uchiha down and the Senju mainly standing. It was a group effort with Tobirama delivering the finishing blow.
Tobirama also died after battling 20 no-names. :/ Not all that impressive. Except for the intent behind that (him saving his team). It's not something to brag about for battle, though.