all of the 03-05’s 9-3ss have the same 2.0 DOHC turbo ecotec based motor.
The linear is rated at 179hp and 195 torque, but typically dyno at 190 whp and over 200 ft/lbs. Underrated to say the least.
The ARC and Aero have the same motor but a bigger turbine and slightly more boost. They are rated at 210 hp and usually spit out a little more to the wheels.
You can tell the difference in 9-3’s by looking at the trunk.
If it says 2.0t (lower case t for smaller turbo) it is a linear.
2.0T (upper case T for larger turbo) it is an ARC
and the aero’s will just say aero on the trunk lid.