First of all Monk has the worst AOE ever. If for example im in a raid with boss i will start by using PB which will leaves it on CD. If i were to do AOE. It'll be Arms of destroyer > Twin snakes > Rockbreaker Total 310 tp used with 180 pot on AOE 100-140 pot on single target because of twin snakes with no GL stacks. It'll take 2.43 x3 with my current non gl stack SS. While NIN can do 300 pot in 2.09GCD x3 that cost 360 TP with my current NIN ss with huton. Not to forget that rockbreaker is in a cone not around you. Also, on the with no GL stacks. Mnks Arm of destoryer benefits from only 5% dmg buff from fist of fire. then 15% for rock breaker with twinsnakes added with fire fist while NIN has 20% dmg from kisses and Huton.
If you were to talk about PB rocker breaker spam U have to at least get a twinsnakes up then do rock breaker spam and that'll waste a 2.41 sec on PB leaving you about 3-4 rocker breaker to do due to PB having 10 sec and GL stacks goes from 2.43 > 2.31 > 2.06 which gives u about only 4 rockbreaker which is 520 pot. while NIN can do 5 death blossom which is 500 pot to aoe in 10 secs or lets say thats 20 pot lower than monk. But if you are considering of using PB i will consider using Kassatsu too. So if it takes away one death blossom which is 400pot kassatsu makes katon crit and katon sits on a 180pot which is 400 + 180pot with crit (im not sure about crit multiplier, was it 1.5x?). This number already over take Monks 4 rock breaker with pb. Also u should consider NIN can do death blossom to the end of Tp but PB only last 10secs.
So if you say whose AOE is stronger i would still say NINJA.
PS. sorry for bad ENG, if there's anything i calculated wrongly feel free to correct me