I can see why closing BT would be less desirable as they just survived an Invasion that could have seen the Continent closed down already. To the other two options offered, closing down Colonies would only offer a potential 108 characters to the other islands, while Dwilight would offer a potential 449 characters to the other islands (Beluaterra offering 251). From those numbers, it seems closing down Dwilight would, assuming the rate of moving is equal for all islands, offer the biggest impact on density on the other Continents (About a potential of 90 per island or 0.72 per remaining regions).
This is not considering retention rates and all that which has already been mentioned, it's just looking at pure character numbers.
