I think Vartann and Catherine look good, but in terms of chemistry and connection, it's Vartann and Sara all the way.
I was watching Dead Ringer again, and there's a part where they're interrogating a cop together and Sara looks at Vartann and he nods at her and she somehow understands what he means by that (even though I didn't).
I think there's a certain melancholy about these characters that allows them to understand each other. They both seemed so personally involved in the Dead Ringer case, it's no wonder she worked with him more than she worked with Warrick. We don't know anything about Vartann, but every time he's with Sara you get the feeling he knows what she's going through. Vartann looks like he's had his heart broken before (avoiding the marriage question?). So every time they're together, you sense a deep connection, even though the characters themselves may not have realized it. You can also sense it in Spark of Life.