The problem i have with this second death star is that they could fly through the whole thing. the first one needed a force guided shot of a proton torpedo to go into a 2 meter wide hole to set off a chain reaction to destroy it. the second one was, fly into it (with the falcon to boot) and just shoot the main reactor. what seems more vunerable to you? while yes it was fairly uncompleted, they flew 'straight through' the DSII. and since only one 'half' was not built, that means their had to be an opening for them to fly into/out of on both sides.
-Darth Simon