I think it’s important for people to define their relationships in terms of specific agreements that are clearly communicated in detail, rather than relying on terms that may mean different things to different people.
To me, “friends with benefits” suggests friends who add hanky panky to the relationship, while “open relationship” suggests a romantic relationship that allows hanky panky with people outside the relationship.
The focus on the FWB is the benefits are extra and the relationship isn’t mainly romantic. The open relationship is mainly a romantic relationship, but allows relations with others.
In both cases, people should be clear about what all the agreements are, for instance particularly where people are having fluid contact with multiple partners, and what communication and agreement needs to go along with that.