Padi AOW is equal to SSI AA
The deep diver speciality is nice, It allows you to go to 40 meters as you say. But the SSI AA or Padi AOW includes other qualifications than depth, like night diving and navigation. But it also depends on where you are diving, how many divesites is deeper than 30 meters? I have done most dives in tropical settings where most sites are down to 30 meters, but visibility often decline when going deeper than 25, temperature drops, and airconsumption and NDL tends to give you a lot more divetime around 10-20 meters then i does below 30 meters. But if you dive more where the interesting stuff is down below 30 meters it is preferable.
I first got the SSI AA then the deep speciality.