Just posted in another thread:

From the Japanese patch notes:

なお、「イフリート討伐戦」では同じグランドカンパニーメンバーのみで構成されたパーティで挑むと報酬にボーナスが追加されます。

That's the sentence about better rewards everyone's talking about.「イフリート討伐戦」 is the part that's over the r30 Ifrit fight. No where in that sentence does 真イフリート討滅戦 appear, which is the part over the r50 fight. So, I think it's safe to say that the reward difference is only for the low-level version.