depends on the spec pal. your oh is faster, but since mongoose is ppm-based, that has absolutely nothing to do with how many procs you're going to see off of white hits. however, instant attacks are still extra chances to proc it at the base weapon speed's (or is it attack speed now?) procrate. so, you're going to want to put it on whichever hand you use instants with, because the amount of procs you get off of white hits are all the same no matter what, but using instant attacks will still get you more, especially if you're combat vs a daggers build, where the ratio of instant to normal swings is a lot more even than it is if you're using backstab (i.e. 2.6 weapon speed is like, with combat potency, probably about 2 white swings per sinister strike, where with a dagger build it's like three or more swings per backstab). this also applies to hemo, if that's still a viable raiding build after the re-nerf they gave hemo or w/e
this is of course assuming you're not some gayass shiv-based raiding build, in which case 1) please respec, and if not, then 2) ya put it on your off-hand first.
orrrr just get your guild to pay for it going on both since any respectable guild has mats coming out of its ass????