Going by the show portrayal of both characters, Pinkamena's just crazy, she isn't doing anything wrong unless provoked. Flutterbitch is just a general asshole. Both of them were both shown to be capable of the same amount of harm.

Given their individual characteristics are pretty much the same, I'll pick Flutterbitch. I can already shrug off the insults she'd throw, and she's still retaining enough sex appeal for me to prefer her.

Pinkamena's hunting and killing tactics are gruesome, and Flutterbitch will taunt me endlessly while following me around. Flutterbitch has an edge over Pinkamena when it comes to following me around since she has wings, but since I don't want to die a horrible death, I'll pick Flutterbitch.