Should i take benadryl (diphenhydramine) or zyrtec for a cat allergy?

Either, though...: Zyrtec is approved for animal allergy. Claritin and Allegra approved for seasonal (pollen) allergy, not pets, though certainly they'd work to some degree as well. Benadryl (diphenhydramine) will work; and though a good proportion will get groggy or sleepy, you may be a person who can tolerate it, and could be more effective than Zyrtec. I wouldn't drive on Benadryl (diphenhydramine) though - it slows reaction times.

Depends: If you only have eye itching, runny nose, or sneezing, either may help but can make you groggy (especially with Benadryl). However if you have asthma or nasal congestion, you are out of luck. If the cat allergy is severe, nothing will work well with continuing exposure and allergy shots may be your best bet.