I don't think it is possible right now since they both use the same method and hook, are you trying to change post content only in a topic? You might be able to use another hook that is only called when inside a topic.
I'm working on a minor "experience" per post based plugin. User posts, it saves experience to an object. The saving works great, it displays fine. What I'm having an issue with is when someone replies to a topic. Everything stores correctly... but since the page doesn't refresh, the post doesnt populate with the updated information UNLESS they refresh the page.
I've tried multiple hooks/filters and can't get any to work. The most appropriate seems to be "action:topic.reply". If I try to make a DB call to grab the object data, the action returns long before the DB call returns so none of the data I need gets added to the returned post object.