Here are the differences between a debater and a defender:
1. The debater is an opinionated person who argues their point in a proper vay, regarding a wide varierty of subjects. A defender is defending one aspect of something he's a fan of. A defender is essentially a fanboy.
2. The debater uses logic and reasoning to the best of his ability for arguing his points. A defender does not necessarily do this (and most do not).
3. The debater can and will argue about anything. The defender only does this when it's an object of his/her fandom.
We're in a message forum so it's likely we all have something we will defend.
However, there are people who will participate in any debate, because they like to present their points and evidence on a subject. This subject they may or may not be thoroughly familiar with.
Someone with a "Great Defender" award will stick up for, say, Lords of Shadow, if the game is being bashed.
Someone with a "Master Debater" will argue about, say, the way the USA should handle torture, whether or not he's a fan or not of torture. It doesn't mean he/she 'defends' torture.
I suppose a Great Defender could also be a Master Debater, though. And vice-versa. But they're not the same.
And yeah I thought they were too similar when it was being pitched as an idea... but I put it up anyway.