Mainly it depends on how much you think chances of a void with RHO are reduced from his failure to bid more, and whether you have entry back to hand. (Will LHO be able to insert honor from KJx knowing you have no entry back to hand?)
With plenty of entries and you don't think possibility of 4-0 is massively reduced, MP best is the deep hook, otherwise Q is best.
Stiff J and stiff K offside cancel each other out in terms of one line being better than the other. So you are comparing Kx onside vs. KJx, KJxx onside.
Quote
I seem to remember @ MP you need to play the Q because this is the only chance to take 5 tricks.
Not "only". Just most likely chance. Also, Q takes most tricks on average. But this is one of the few strange combos (assuming 4-0 not reduced, enough entries), where MP-best is not the same as max trick line. Q is max trick line mainly because if RHO has stiff J, you gain 2 tricks (since if you hook and lose, next play is A), while if RHO has stiff K, hooking deep only gains 1 trick, which makes up for the overall lower frequency of the hook Q line. But at MP vs. pairs taking the alternate line, you don't get extra MP for beating them 2 tricks instead of 1 trick, so frequency of the various combos is all that matters.
I think though, given that both opps bid but didn't compete 3/3, that RHO is less likely to have either void or stiff than normal, enough to give Q the edge on the problem as presented.