With Jaime's re-emerging conscience, I don't see him killing Cersei out of vengeance or convenience. He won't kill her for vengeance for two reasons. He'll hold out hope for Cersei to change... like he is doing. He's believed that they were one person in two bodies. Second, he's striving to put the realm before his own interests. He may well kill Cersei, but it would be because she's betrayed the realm, not because she's betrayed him personally.
Also, I don't see Jaime killing his sister merely because it's convenient for him at a certain time. This is what he did by lying to Tyrion about Tysha, what he did by tossing Bran from that tower, what he did by slicing through Robb's bodyguards, and what he did by giving Catelyn his oath. It would be convenient for the Martells, the Tyrells, and the Sparrows to kill Cersei... and they might all have a go at it. You know, I think that Jaime now needs a better reason than convenience.
On the other hand, I think vengeance and convenience are the two most likely reasons that Tyrion has for killing Cersei. If he gets any chance... ANY... he'll take it. And he'll gloat the entire time. Of course, if Tyrion is allied with Dany, then Cersei's death would be also for political expediency... but make no mistake, Tyrion will greatly enjoy it.