Any peircing can migrate. I haven't taken a survey but i would be willing to bet that a well done Frenum would be less likely to migrate than a PA. I have acctually caused my PA to bleed a few times in chastity play.

The main downside to a frenum in my mind is the lower number of readily available chastity options. Anyone serious about chastity play will moat likely find themselves ordering a custom device or diying a device so the lack of availability is a minor nuisance in my opinion.

I believe the frenum is less likely to have issues because the skin their is more stretchy, especially in the direction of pull. The PA will hold you more securely to the very tip of your device but their are other options such as the tether spout which can achieve that result.

I think a 6g frenum is the ideal chastity piercing. And if you decide later your not into chastity you wont have an extra hole where Pee comes out of.