The thing is, if you firm Plymouth you might not be able to get a place at Cardiff because the course might be full.
It would be the safer option though, because Plymouth might not accept you with lower grades if you don’t firm them.
If you really want Cardiff, I would firm them, there is just a slight chance you won’t get a place at either if you don’t get the grades. This is unlikely though, because Plymouth would probably accept you without ABB anyway.