you should do the topic you're most interested in -- you'll be naturally passionate about it which will shine through the delivery of your speech & it will also allow your writing to have a 'voice'
I agree with Swagadaktal. From experience, in public speaking competitions and last years oral SAC you don't necessarily have to argue a topic that you are passionate about. One of the fundamental aims of this SAC is to convince your audience to concur with your view - persuasion is a skill, a skill that can be learned. The more you practice a speech, practice intonation the more convincing you'll sound.
Even though teachers may say otherwise, the more a topic is used the harder it is to standout and engage your audience (unless it outshines the other speeches on the same topic) and subliminally even, the teacher will be not as intrigued in your arguments (most of which are likely to have been covered by other students) than if you chose a unique topic with fresh arguments and ideas.
Ultimately, no matter what topic you choose you should be able to imbue it with a 'voice' however vague and abstract that sounds.
In my opinion, choose a speech topic that your audience may not have even considered - get them thinking.
Good luck