It does matter. Content based discrimination is allowed under specific circumstances, wheras, viewpoint discrimination is never allowed. Such as the transit authority can prohibit all ads for political parties, as long as the forum has never been open to political speech, but once open it cannot thereafter prohibit any politician from advertising based upon there political affiliation. Of course there are exceptions to this as well, but in general this is the rule.
Actually, that's not quite right. When looking at laws that restrict speech, these laws can be either content-based or content-neutral.Content-based laws reference speech by their content. Content-based laws can be either a subject matter restriction (Jacy's first example) or a viewpoint restriction (Jacy's second example).From a practical standpoint, it doesn't really matter whether a law is a subject matter restriction or a viewpoint restriction because both are content-based. However, on a law school exam it helps to know the difference.