Due to all the people saying she'd replace Anya or stay with Xander, I now hope that if she stayed in Buffy she'd have been killed.
No, I'm kidding, I'm just bias towards Anya. Moving on, Cordy was one of my favourite characters on Angel, but only because of how much she changed from being in Buffy. I think if she'd stayed on Buffy, she'd have taken a lot longer to change from the shallow Cordy of seasons one through three. She wanted to become an actress, but ended up working for Angels Investigations, yes? And I think helping the helpless was what completely changed Cordy (That, and falling in love with Angel, obviously 😉) and whereas if she'd stayed in Sunnydale and attended UC Sunnydale she may have stayed associated with the Scoobies, I don't think she'd have been a vital part of the gang - She'd have still been human, and I think at some point or other she'd have probably been killed off, not necessarily, but I think it'd get to a point where Cordy seemed a little pointless.
Maybe it's just because I'm a diehard Anya and Xander/Anya fan, but I don't think if Cor had stayed she and Xander would have gotten together again, or she'd have replaced Anya. Anya actually was helpful, as she had demonic contacts, and after over 1100 years of life, she had a lot of experience; Cordelia didn't. And, Xander and Cordelia? Come on, it was a high school relationship, did they ever even say 'I love you'? (If they did, I'm sorry, I don't remember it. I try and blank their relationship out of my mind. 😛) Xander and Anya had love; real, powerful love. I mean, they were going to get married, they lived together...Xander and Cordy never even slept together. I just don't think there would have been much point in the character of Cordelia long after season three, personally, I think that's why she was moved into Angel.