Audit or B in class when applying to grad school (mathematics) Does a B in a class look worse for graduate school admissions than an audit, given I'm not expected to have the class in the first place?

More specifically, I'm studying abroad and taking five math classes. When I go back to my home institution, by the time I graduate I'll have exhausted all the math courses there, so the maximum math courses I'm "expected" to have is those plus four (four classes being a typical semester).

I have the opportunity to change a class to audit, even after I know my grades, so my question is, if there is a class I might get a B in (functional analysis, if you care), do you think having a B would look less impressive than not taking the course (or does audit risk the same sort of judgement)?

Does my question make sense? Does a B look "weak" when applying to grad school (even in comparison to nothing)?
It's better to have documentable (whether in transcript or by letters of recommendation) exposure to more substantial mathematics. Especially in the U.S., the default undergrad curriculum in mathematics is far too thin for grad school. Without taking initiative, which necessitates not-the-routine channels, you'd not be ready for grad school. Naturally, the irregularities in the not-routine channels generate grades whose meaning is unclear, and so on. (And, no, summer "research experiences for undergrads" do not typically add much to one's scholarly preparation, although they may fire one's enthusiasm.)

So, for me, when I see "functional analysis" on a transcript, I don't really care what the grade is, nor whether it's "audit" or not. Just the  initiative to engage with it.