In general defense buildings are a little spotty in ISG, as its very easy to get past them (attack an asteroid, or simply the fact that attacking for initiative denies you the bonus when you are trying to defend). That said, I do use the Airspace Defense Nexus line on occasion....however, I almost never use the Planetary Shield. Its not the cost, its the slot...infrastructure slots are precious, using one on defense I can do, but two is very tricky (especially in late game when you want warp interdictors and the like). So my thought, combine the two lines into a super solid all around defensive building. The go to place for defense, if you want to spend a lot to defend your planet really well....here you go. Otherwise, I don't every see myself using both of them in a competitive game.
