If I was going to join one of the two it would be Phi Beta Kappa, because it "seems" to be more "prestigious," but personally I don't think much of the requirements for either one. Maybe it is different at other schools, but the requirements to be invited to both are a joke at my school (at least in my opinion).
-Phi Kappa Phi: Top 10% of Seniors, or top 7.5% of Juniors
-Phi Beta Kappa: Junior with a 4.0, Senior with a 3.5. And a "diverse" list of classes (2 foreign languages, 2 math classes (

), etc (basically the core requirements to get a degree

)
If Phi Beta Kappa changed to allow only seniors with a 4.0 or higher then I would be more inclined to join as it would have more prestige (imo), but 3.5 is just a joke.
And our PBK is $65, one time. Which is not as bad as PKP, but there are other things I would prefer to spend $65 on.
Also, getting a letter from PBK that starts "Dear Honoree" does not impress me either

And PKP did not even send a letter, they sent a mass e-mail!
I have until Monday to decide on PBK, so I will ask some professors about what they think and then make a decision. I already asked my adviser, who is also the graduate chair, and she said that she doesn't think any of these honor societies carry much weight anywhere.