The interesting choice is inside the objective
Both games turn trick-taking into a shared problem: winning the biggest trick is useful only when it helps the crew meet its assigned conditions. For a group already interested in that idea, the difference worth choosing is the form of the task. Quest for Planet Nine repeatedly asks how a particular card can reach the right person. Mission Deep Sea can ask what someone must collect, avoid, predict or win across several tricks. Those questions direct attention differently even before the space or underwater setting enters the conversation.
A named card gives Planet Nine a clear centre
In Planet Nine, a task card depicts a specific playing card. The player who takes the task must win that card in a trick. The commander starts the allocation and selection continues clockwise; task tokens can add requirements about the order in which objectives are completed. The fifty-mission structure gives the group a sequence to return to, although the rules allow missions out of order.
That makes it a good candidate when your group likes a concrete shared question: how will this card reach its owner, and can the other objectives still happen in the required order? The next deal can change the answer, so following numbered missions does not mean memorising one fixed set of moves.
Deep Sea asks the crew to assess a combination
Deep Sea task cards carry difficulty values for different player counts. The crew draws until the values reach the mission's exact total, skipping cards that would exceed it. Conditions range beyond one target card: a player might need an exact number of tricks, no cards of a stated value, or a particular relationship between captured colours. The allocation becomes a discussion with the cards, not unrestricted verbal planning; the rules still restrict revealing your hand.
That variety can suit people who want to reconsider what success means each time. It also adds work: the rules explicitly address impossible combinations and when replacing a task or redealing is allowed. More varied tasks should not be mistaken for a promise that every initial combination works.
Keep the route, or change the problem
Choose Planet Nine for the appeal of directing named cards through a progressing set of missions. Consider Deep Sea when varied conditions and judging their interaction are the attraction. Its retry rules even let the crew keep the tasks or draw new ones. Both retain limited communication, so neither is the obvious answer for a group whose main pleasure is freely explaining every card in its hand. The useful purchase is the problem the group wants to return to, rather than the larger count on the components list.
Sources & further reading
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