Depends what exhaust you have on there. Original or after market, Full system or only a slip on.
By my experience the juice box works best with 2 Brothers exhaust, since it is made specifically for their systems, can't say anything about P3.
As for programming, My JB is an older model and has sensor push buttons and is easy to use, probably all depends also on what you like best.
I believe it is always a personal preference which one of the two options one chooses.
I've been debating in these for awhile seeing as thy don't make a pc V for the 06 636. Pc III has add ons that's nice where juicebox does not but if you get the juicebox pro you can have 10 different maps programmed and switch them on the fly supose to be. That's kinda nice to have specially if ur trying to get gas milage on a trip then u have some ricer in a civic that wants to play just hit the button to have more power tune lol. Juicebox pro has pretty much no add ons thou. I think u can add a quick shift but that's it. Oh and juicebox pro you do not need a computer to program it. Prob easier with one but you do need it for the pcIII for programming. I don't think ull go wrong either way like always pros and cons d the user. If you want add ons like screen display and whatnot then I'd say pcIII if you don't care for add ons and more performance I'd say juicebox pro. Also juicebox and the pro version WILL accept powercommander maps not just two brothers !