A) Staal (3.9 this year) either walks for nothing next season or is re-signed for ~6mil per season.
B) Leddy (2.7 this year, which is less than our 3rd pairing D Kevin Klein) is a RFA who can't walk for nothing and can either be locked up long-term if he fits in here, or traded, or taken to arbitration.
Scenario B has a lot more positive options/outcomes.
Not to mention Staal has likely played his best hockey already, while Leddy at 23 still has room to grow and develop as a defenseman, and already provides more offense than Staal by a wide margin.
The only thing Staal has on Leddy is that he's better defensively.