The difference between a 3.9 and a 3.96 is pretty meaningless.
I would advise you to take the B+ as a letter grade if you have the option. School opinions are changing. Many say they will accept P/F grades without prejudice, but some say they will only accept them for pre-req's if the P/F grade was not optional, but recommend students take the letter grade if they have the option.
Irrespective of policies, a good grade (and a B+ counts as a good grade) helps showcase something good. A "pass" grade is neutral. Since there is no grade attached to it, an admissions committee can't use it to judge your work in that area- they'll pass it by and rely on the rest of your application. This may not be a bad thing, but in general the more pieces of "strong" evidence you have, the better.
All of the above said, I doubt either option will have any meaningful effect on your application. I will urge you to not overly stress about your GPA. Good grades are important, but they don't have to be perfect. Striving for a 4.0 may well cause you to avoid other opportunities that will be far more meaningful and improve your application a lot more.