So does it all come down to the strongest bond he has?
I believe that Buffy is his one true love and nothing could ever stop their bond. She was the one that had mattered the most to him through all of those years...
Cordelia and Angel are close friends, but I don't think their bond isn't that deep. They barely talked in the first 3 years they'd known each other and even though their friendship grew, they've only been together because they work together. They were never truly in love, IMO.
Angel's relationship with Darla is a very close one since they'd been together for 148 or so years. Plus, she made him. Then again, he had never wanted to become a demon. She's also had so many chances at life, so I don't think he'd choose her.
Angel, being the hero that he is, would probably refuse to choose and try to find another way around it...