I'm trying to make the same decision now as well for my ret set that I'll probably never get to use. But, how can I leave Jinrhok sitting in my bank without a shiny enchant on it? From what I've read, Mongoose > Executioner > Savagery until you get large amounts of static armor penetration on your gear, at which point Executioner is better. This also depends on what kind of armor your target has (I think those calculations were done for a standard raid boss with something on the order of 7000 armor).
For PvP, I would think that Executioner (or maybe even savagery or agility) is better, just because you don't often get to stand on an opponent and go to town on them. You often have to move around or heal, or dispell or something, and you might not be able to put mongoose procs to their full use.
I sure wish there was a melee equivalent to Dr Boom. I can take my warlock out there and test my DPS really easily. Too bad WoW was built to massively favor range DPS at almost every turn.