Contentious Vs. Controversial

When applied to things like issues (that might generate contention/contentiousness/controversy) it's probably pointless trying to impose some kind of distinction.

But when applied to people, usually a controversial person is someone that others argue about because they have different opinions about him (not necessarily because of anything he says - perhaps some people just don't like the colour of his skin, or the fact that he had a sex-change op).

On the other hand, a contentious person usually means someone who habitually argues. In principle, such a person may generate very little controversy (everyone else may be in complete agreement that he's tiresomely quarrelsome over things no-one else really cares about).