I'm actually quite specific in my layout. If the asteroid has poor stats (core energy less than 100, others less than 40%, &c.), I plant one dyson tree and make the rest defense trees, just so they can be static defenses against enemy swarms. If the asteroid has appreciable stats, I plant more dyson trees than defense trees, but both will be present if possible. For example, if I can plant five trees on a nice asteroid, I'll put down two defenses and three dysons.