Alright I will help you out a little here, my main is a Juggernaut Tank, all ops cleared, 100 valor, I got 2 powertech's on the side, one dps and one tank. Think I can objectively answer your questions.
a) after the 2.0 nerf to pyro and the buff to vengeance, definately the juggernaut (translates into vigilence for guardian)
b) for pve, the tanks are pretty much equal, the dps severely favours the guardian
c) guarian is the best main tank for pvp, best guardian for healer and best protection, VG is rarely used in competitive pvp
d) I have 2 PT's 1 Jugg, I never switched mains but felt like lvling 2 PTs though, I'd still go for the jugg, specially in tanking, but also in dps
e) PT/VG tank and dps are brainless, require zero effort to play, jugg has more skills to use, more cooldowns, more buttons to press, the same is true for dps specs but less so than for tanks.
If you want more info just shoot me a whisper Im more than happy to answer anything