Is it better to fly or to be better at beating spell resistance?

I’m a sorcerer planning for down the road.

I have the Celestial bloodline.

At level 9 I have the option to get the bloodline ability Wings of Heaven:

Wings of Heaven (Su): At 9th level, you can sprout feathery wings and fly for a number of minutes per day equal to your sorcerer level, with a speed of 60 feet and good maneuverability. This duration does not need to be consecutive, but it must be used in 1 minute increments.

In place of that I can get the bloodline mutation Blood Piercing:

When you cast a bloodrager or sorcerer spell that deals damage, creatures affected by the spell reduce their energy resistance and spell resistance against the spell’s effects by an amount equal to your Strength or Charisma modifier, whichever is higher.

Is it better to get the Wings of Heaven, or to get Blood Piercing and the spell Fly at level 9?