Extension:MediaWikiAuth (MWA) is not actively maintained (so it may break unexpectedly with updates to MediaWiki and/or Wikia) and has been highly problematic in our experience. Extension:StubUserWikiAuth (SUWA) is written and maintained by Ciencia-Al-Poder and used on Wikidex and the forked Runescape wikis, so breakage is far less likely and it is known to be performant.
MWA actively imports user preferences on account import, meaning it relies on the remote wiki authenticating the first login. Because of this, closed and blocked accounts cannot successfully be imported. It can import user groups (which we have enabled for all groups admin and below) and watchlists (which we have disabled in an (apparently unsuccessful?) attempt to help with import issues). It does not pre-populate the user table, meaning a user account is only created on first log in/import, which in turn means that account imports can be seen on Special:Log/users.
SUWA prefills the user table with entries for all users who have edited the wiki, and logins which import an account are not authenticated against the remote wiki. This means preferences cannot be automatically imported, but might allow closed/blocked accounts to be imported (this needs investigating, but frankly I'd be surprised if it's actually the case). It also, and more importantly, means imports are not reliant on the remote wiki. However, imports are not visible in the log or anywhere else on-wiki; the only way to know if an account has been imported is if it edits or performs logged actions.
At some point we have to assume that the majority of people who are interested in importing their account from Wikia will have done so already, especially given the long-standing issues with successfully importing accounts using MWA. In particular, basically all of the active administrators have already imported their accounts, and virtually all editors who were active and prolific leading up to the fork were contacted to see if any of them were interested in moving with the fork. In the longer term, it's probably not a good idea to assume that Wikia won't make any breaking changes to the account login procedure; if and when they do, further imports will be impossible until MWA is updated, which requires the update to be possible in the first place, and for someone to be available with the technical ability and the interest to update it (in particular, the person who last made large modifications to MWA, Skizzerz, is unlikely to make further changes without being paid).
These combine to show that there is a diminishing benefit to using MWA over SUWA into the future, and that at some point we will want to cut potential losses and switch; the appropriate crossover point may have already passed.