Antagonistic posts are ones that purposefully antagonize certain people or bait other members into arguing... things like "I hate all men/women, here's why"... with questions, there's nothing really being asked there, it's just a tiresome rant from someone calling out why they don't like someone or something.
Offensive posts include content that may be seen as degrading or distasteful by most members. Racism, sexism, homophobic comments, personal attacks, etc.
It's true they're both similar. If you report something and aren't sure which one to choose, just pick one, and the Admins will take it from there.
"antagonistic" is one being hostile or trying to create a form of negative reaction. That or it is openly hostile towads a group. offensive simply means the content is offnsive to some. For example asking in explicit detail about scat (poop fetish) or trying to offend people.