since this is "prime" itachi, im guessing this is healthy itachi. if so itachi takes scenario 1 low-mid diff as he is smarter than obito so he would figure out the mechanics behind kamui and therefore outsmart him by tricking him to warp an explosive clone. if itachi uses amaterasu on obito while in the process of being warped he will be forced to use intangibility therefore he will be unable to warp itachi. amaterasu overpowers any large scale katon obito has.
susanoo in its v2 form tanks any head on attack obito can offer. itachi can also summon this version of susanoo very quickly as zetsu already stated that lighting moves at 1/1000th of a second and itachi was able to put up susanoo at the moment sasuke struck him with kirin, so any last second activation of susanoo works if it becomes a last resort. obito has to become tangible to hurt itachi so he also leaves himself open for a counter attack. if itachi chooses to preserve chakra he can engage obito in cqc using a ribcage susanoo with both arms fully formed. i dont see obito winning in a straight up cqc fight against itachi under the conditions i just mentioned even with his chains and intangibility.
if itachi lands tsukuyomi obito is done for. if he gets hit by totsuka blade hes done for. if he gets hit by a susanoo punch to the head he would most likely be done for. itachi's counters to kamui are amaterasu and susanoo while obito has no counter for susanoo and amaterasu except to use intangibility when attacked by them but itachi would eventually blitz him with totsuka. itachi just has too many ways to one shot obito if he is hit by his attacks while obito doesn't really have a way to do anything to itachi while susanoo is up even in v2 form let alone v3/v4 with yata.
as for scenario 2 i see itachi winning as he still has explosive clones and is smarter and has better taijutsu and genjutsu.
