MongoDB best practice - array vs. new documents -

i have system users , competitions in mongodb. want users able sign competitions. best practice:

• to have third collection named "competitionsignups" signups saved in separate documents - references both competition , user.

• to have array in each competition document references users has signed it.

• a third solution??

edit: primary operations are:

• select competitions user signed for
• select users in competition
• select users signed given competition, , hasn't used account given time period. can unregistered competition. (the user documents has parameter named "last_use_date").