1990 Toyota Corolla Having Alternator Problems Please Help!

Hi, I have been having some problem with my alternator lately and now it decided to call quit.
I already tested and everything so its a definate because the battery is brand new also.
I went to my local autoparts and I found one for around 80-110 depending on brands and what not.
Then the gentlenment said they have 2 different alternator for this MAKE and MODEL.

So my main question would be.. how do I know which one to get?

Thanks a ton guys!