As described, the winner will be coalition X, as the combined coalition has more points than you do. Both members will be awarded the coalition win statistic and will each get the coalition victory reward. You will receive second place and get the second place reward plus VP, which may be more gold than the coalition reward.
If you join the coalition, make sure you wait until after the next day change before retiring, as you don't want the retirement to be processed before your coalition joining is recorded in the next issue of the newspaper.
