Most people say Felicity is too frail to be useful. I personally disagree, since Death Sentence gives her what she needs, and Z6/Z7 equipment makes her even less frail.
However, I still use Roald since I'm a speed Bio and another speed-based character would be redundant.
Go with whoever rounds out your team best, but if it's Felicity make sure you give her Vitality a boost with equipment.