Well, I don’t even know, the situation is very twofold, depending on who you need Painwheel or Robo-Fortune more. As I understand you need characters for defense, here's what I can tell you: Twisted mettle is much scarier in defense (if her Marquee Ability is pumped into TAINTED BLOOD), but she is weaker in diamond than Blue screen (both in terms of HP and damage) . If I were you, I would pump Twisted mettle first (plus, she can show herself in the attack at least somehow because of the second Signature Ability).