I don't think that SGA had a better year than Tatum in his second season. SGA tops Tatum in box metrics, but that is mainly due to role: SGA is OKC's secondary ballhandler in the starting lineup and gets to make on-ball plays considerably more than Tatum, which will pump up box-based metrics as long as his efficiency isn't poor. Tatum also has a significant edge on defence, which is captured in non-box metrics such as RAPM (albeit NPI, although it still holds true in multiyear samples that are much more reliable), which has him ranked 12th in multiyear defensive RAPM from 2017-2020 while Shai is ranked quite low, which makes sense given that Shai is an alright man defender but a problematic team defender. I think Shai is in a similar range compared to Tatum in his sophomore season: both are top 50ish players.
As for the potential of both players, I think Moranis is spot on regarding who has the higher ceiling even though I don't think either player has clear MVP potential, maybe Tatum has a shot but I don't see it for Shai.