So im looking to buy a new cat for my 91 Camaro RS in hopes that itll pass smog after. I have a question about the cat I want to get, if anyone has any info out there.

http://www.rockauto.com/catalog/x,ca...WALKER%2B80544

So if you look at part #80544 it says to use until depleted then use the #80874 which is a couple dollars more expensive. Whats the difference? theyre both California smog legal but should I just go ahead and get the 80874?

I would just go with the 80874.
When I replaced mine many years ago a shop had to order it and install it using this little gem of paper.
There's more to the form but not necessary to show .You get the idea.

"Depleted" refers to mfr stock, not the converter itself. I.e. the one part # is being discontinued and replaced by the other, most likely for reasons of convenience to the mfr, having nothing whatsoever to do with your situation. So it probably doesn't matter which one you get.