The WRX has alot more time and money into it, better parts (fortune auto 510s vs Silvers for example), more seat time, etc.
I would say the WRX would be better suited for a track day in their current state, but I have autocrossed both cars on the same day and the BRZ with little seat time is competitive with the WRX as is. The WRX needs to be driven "like an angry teenager" and the BRZ is much less forgiving but also much more nimble. Alot of that probably has to do with me just being use to the WRX.