there are many sides to this. animal cruelty via establishments (zoos, circuses, etc) and pet ownership are definitely a human problem. meanwhile species being endangered or extinct is simply a part of how our world works. however, more species are becoming endangered or extinct more often because of human interference, be it hunting, habitat destruction, or whatever. also, animal cruelty is usually out of selfish reasons and/or lack of respect for the animal, and animals being hurt in this way are intentional. animals going extinct and endangered can be affected without humans meaning to - for example, someone buying an area of land and building massive settlements on it, therefore destroying the habitat and inadvertently killing many of the species that live there.
they're both obviously serious issues, but i have to say that endangered species are more serious. animal cruelty is obviously horrible, but it's not usually an endangered animal being purposefully neglected/abused. it's also an issue that can be more pinpointed and easier to avoid, since it's also usually with domesticated animals (be it pets or lab animals or whatever). the owners can be charged and sent to jail, animals go to the vet. establishment revealed to have been abusing performance animals, establishment faces consequences (like the circus removing elephants from their act). but then endangered species is a widespread problem, and not just with animals, but plants as well. and some of these endangered organisms might be harmed out of necessity - some people might poach because they can't get food anywhere else.
but yea like i said before both are very serious, and i dont like either of them. but if i were asked to join a campaign to help end one or the other, i would probably choose endangered species.