meh... avatars are already sitting about 55% -PDT naturally because of Stout Servant.....throw on Ducal Guard and they are capped on PDT at 50% hp in abyssea. Not sure just how useful a -PDT staff would be for SMN, except for outside of abyssea I guess. Even then, you may be wanting to use your -perp staff as the main (esp. if you haven't completed Caller's set yet for the extra BP reduction, and -perp, etc) and just swap into your pet MAB when nuking--in which case, you wouldn't even be using the -PDT in that case and you could just put it in your satchel/sack.
But, if you're going to carry them all the time in your regular inventory, your best bet would be to complete one and leave the Shareeravandi at +3, whichever one would benefit you more (only completed one SMN staff so far, rest are still at +3 stage, so not sure yet if MAB or PDT gets another bump or not). Personally, I would lean more toward extra INT/MAB if they do get another tier, seeing as with -12 PDT your already @67%.
