I'd go with the angelfish if you want a larger fish. They will still possibly eat guppies though. Nothing wrong with a good ol' scalare. I like the koi angels myself. Would you get a single one or a group? They can get a little aggressive if they spawn so just keep that in mind.
Discus should be kept singly or in a group of 5+ and in a 55 that wouldn't work for them very well.
If I got angels I'd keep at least 5. I am off from school tomorrow, and my lfs told me 2 weeks ago he,d have new fish in today, so we'll see. I'll post pics.