IMO it's unfair to compare FF2 to SR.
And it's unfair for Superman Returns.
1)
FF2
Critics Tomatometer 36%
Users Tomatometer 61%
SR
Critics Tomatometer 76%
Users Tomatometer 76%
2)
SR was not the Superman movie that a part of the fans was waiting for. I agree that it lacked action and the story was too similar to the Donner movie. But the movie remains good.
The story of the worst superhero movie ever made is pure crap. Superman Returns is a good sh movie.
A lot of haters wrote that the box office numbers proved that SR was a failure: it did only $200m, so it is clear that it was a disappointment.
For the haters SR did only $200m because it sucked.
If you say that SR was hurted by the terrible release date and by the bad marketing, they will write that you are a stupid fanboy.
For example, LexLives (a famous troll in the SR forum), last year predicted a megasuccess for FF2 (+400m ww) to prove that Superman is a dead franchise or that SR sucked or that a reboot is needed

.
But THIS YEAR, three movies with the potential of $400m have done only +$300 because of their bad release dates: Spiderman 3, Shrek 3 and POTC3 were three huge movies released in May. Even FF2 was hurted by the huge competition.
3)
FF2 is a sequel.
When FF was released this forum was full of "Amazing movie! It is a great movie! It's better than Batman Begins!"
IMO FF was a barely decent sh movie, and it was considered a success only because its $100m budget.
But this is my opinion, for the vast majority of this forum FF was a great success.
With FF2 they increased the budget of $30m and they are doing at least $20-25m less than the first movie (I doubt that it will do better overseas).
IMO FF2 is a good movie, far better than the first one, but IMO they did a mistake increasing the budget.
SR, which is considered in this forum a sort of disaster, did $391m ww (a great result for a sh movie). At the WB it seems that they have decided to do an action packed sequel with a reduced budget ($170-180m).
IMO it is the right choice and I'm quite sure that it will be a b.o. hit, because I think that the situation of the new Superman franchise is far better than what we think. A lot of people hated it before the release date, and the expectations of the spiderman numbers were really insane.
SR was easily a $220-230m movie without the release of POTC2 after a week and a good sequel can easily be more profitable.
I think that the perception of the reality sometimes is distorted.