Oh no! I think you should keep both the black and the seafoam... I'm not loyal to one brand either, and I have my eyes on an original 2.55 flap since it's so Chanel!
Honestly, the black BBag in the first or the city is sooo classic, and the seafoam is just rare! If you sold it, it would be really hard to get back. How do you feel about dropping the Chanel bowler - that one will still be around in a couple of years I think, so if you're worried about something disappearing, it's more likely the re-issue.