I've worn it both ways, but historically preferred the harness under the PDF, mainly because then the shoulder straps are tight on my shoulders.
Now I have a fancy pants Zhik T3 with the safety emergency detach/eject velcro. Fits nice, in particular since my wallet is now considerably thinner –
Anyway, if I ever need it, the eject velcro won't do me much help if the harness is under the PDF, because I can't easily detach the shoulder straps nor get out of them. So _over_ is the safe option. But now the shoulder straps "fall off" my shoulders anytime they're not under tension.
Thoughts?