I only really know stuff about the A/R ratio's....
thats assuming your ignoring the wastegate tho right?.......otherwise they will both produce 9psi. ( assuming that the A12 can flow enough exhaust gas to produce 9psi with that exhaust housing )
A bigger A/R on the front will allow the turbo to flow more air at the same boost pressure. tho it will make the turbo lag more ( theoretically....not sure if it'd be enough to notice while driving )....for the same reasons you get more lag with a massive intercooler when you don't need it.
and as pro240c said...work backwards....
heres a link with some real good examples of how to do some engine calculations ( not really on the turbo's.....but very interesting ).
( reccomend you read the rest of the article too )
classic example of working backwards......not only did they deisgn the engine that way....they then designed the car around the engine!
1970 Datsun 1600 CA18DET - SOLD!
1970 KP510 SSS Bluebird coupe