I agree with Pyrexia!Because you already have a prune bag, I'd suggest going for the Birkin in BJ. I think it will provide some nice color to your collection.
It also depends on what you wear a lot of though - will BJ fit into your wardrobe as easily as (I assume) prune does? If not, then perhaps going for the prune kelly would be a better choice (who says you can't have too many of the same color anyway?)
You should definitely be able to fit in a Kelly longue in a 28, but I'm not sure if that will leave you with much space for anything else.