I think the ones who are saying Itachi are rating him by feats he has displayed as an adult. Think of it this way:
Both Both of them awakened the MS at around age 13. So they each have had a year to hone their abilties. ANBU for Itachi and under the flag of the Akatsuki and secretly Madara for Obito.
In that year:
Did Itachi acquire the yata mirror and totsuka blade? Was he able to use Amateratsu, Tsukuyomi, Susanoo, etc?
Obito has had a year to perfect Kamui.
Now let's compare the 2
Kamui can teleport Amateratsu away. Not sure about Sasanoo but their are other ways around that. like kamui through the ground where Itachi can't touch him kamuing inside or underneath susanoo and Kaumi Itachi away.
Tsukuyomi would probably Itachi's best best provided Black Zetsu doesn't interfere and or Obito doesn't break it (he is an Uchiha after all)
Lets not also forget Obito has access to Mokuton and at the age of 13 his MS was strong enough to controll the Juubi and face Minato the 4th Hokage at the time.
Itachi was not that skilled at 14 as he is now so you need to keep that into account. The numbers stack in favor of Obito for this fight.