I think the fans are 50-50 on that. Fans of the older games usually prefer Chris, since Leon had no personality in RE2.
I honestly don't see Capcom making them kill each other. I think that, while they're fighting over Ada, someone else will kill her (for example, Jake or Alex), or she'll run away.
As for "Ada's" role, I think she's a triple agent, working for Alex as a villain, but giving informations to the chinese government, while her true heart lies at the Organization. Either that or she's working for the Organizations as a villian and giving info to the government, that's why Crhis wants to kill her (she is responsible for the death of his teammates), but Leon wants her alive (she has vital information about the terrorists).