This is just what I have gleaned from reading stuff on here, someone with more expertise may give you more info, try reading the tech articles on here, they help a lot.
The IHI is smaller and will spool up faster, but cannot produce as much boost and the air will become superheated at around 18psi. I guess they begin to spool up at around 2500rpms.
The T3 is larger and will take more time to spool up (begins at around 3000rpms). It will be good to produce up to about 24psi. Much more potential for power here.
these figures should be assuming with a stock setup. In stock form, the figure I have seen around here is 1psi=6hp(I dont know about torque). Looking above at the figures here, you can see a 36 hp difference between the two turbocharger. Headers, porting(either the head or manifold, or both) can increase power and reduce spooling time. Intercooling is highly recommended for these.
I have seen where people run 18psi boost with a T3 unintercooled. I dont know if they have any problems with detonation though.
Now it is time for someone more knowledgable than me to step in.