When should I use a raft, when should I use a brim?
A raft will allow for better adhesion for the whole print as the raft attaches to the printing surface and the print attaches to the raft. Rafts go all the way under the print and consist of multiple layers, whereas a brim is only 1 layer and on the outside of the print. Rafts are normally harder to remove than brims because of the increased contact with the print.
From my own experiences, the brim does not help a lot with layer adhesion as it is only 1 layer. I normally use a raft when I need a nice looking 1st layer that is not on the bed or when there are not enough contact points.
I find brims useful when you do have a lot of contact between the bed and the print. If you have a relatively flat object, a raft is not needed but a brim can give you something to use help separate the print from the bed. For example, I recently converted a 2D logo image from a non-profit into a simple stencil they could use for tracing. Very flat, lots of bed adhesion. The brim gave me something to use to pry up the print without damaging it. It's also useful for small boxes or trays. I use a brim whenever I don't need a raft.