Forums as communities or topics?

I have generally been viewing forums as topic based. By this I mean the forum you post to is based on the topic at hand. Recently I have seen some posts using them more as communities i.e. a filter on who views the post and comments on it. If I post about something Apple related should it go on the Apple forum because that is the closest topic match or the Android forum because arguably that is the community I'm in?

So , communities or topics?