Unfortunately that's kind of like asking what the difference in rank is between a Knight and a samurai. Poly is not monolithic and friends with benefits is itself a fairly new and evolving phrase, but from my point of view, a secondary poly partner is in a formal arrangement and fwb is informal. A poly partner is more likely to have constraints about what they do with others and has stronger emotional attachment. An fwb is a "we can both do whatever but when convenient we have sex"