I'm going to order some magnaflow catalytics for my 97 camaroz28.
But there are two options- one comes with the air inlet which is a piece of pipe that's threaded on the end coming out of the converter, and the other is a threaded hole. Which one do I need? Or do they both serve the same purpose for the o2 sensors.

 

You do NOT want the catalytic converter with the air inlet pipe. That is for the 3rd Gens, that have a belt drive AIR pump that runs continuously, and supplies air between the two beds in the cat. The 4th Gen has a monolithic cat, which does not require the intermediate air.

Appears you are looking at their "universal" cats, and those are the ones that have the threaded boss for the O2 sensor. You would need a plug for that hole, since you already have mounting locations for your sensors.

I've got dual Magnaflow/Car-Sound 3" cats, and its a model with no air tube or sensor boss.