Basically, the Pittsburgh Structure line was designed (and for a time, built) by Monorocket, who had a reputation for making extremely solid cabs with very ample power supplies. The Structure 420 shows this, both in its built quality and the fact that you get an overload and short-protected 6000 mA on each 12V bus, and 5000 mA on the +5. Compare this with the v.3 power supply on the A-100 p9, which is 2000 mA on the +12, 1200 on the -12, and 4000 mA on the +5V rail.
I think you should seriously consider Pittsburgh's Structure 344, though. 5000 mA on each 12V rail + 3000 mA on the +5, same protections + extra RFI filtering, and you get a really well thought out 1U utility row with some very sensible utilities provided. More expensive, yes, but the extras + the form factor (more like a Doepfer p9 in size) make a good bit of difference.