How do I replace my organisation's event video camera?
Firstly, that camera has a 29 minute 59 second recording limit. If your speaker goes for longer than this it may not be suitable. Apparently this is due to some countries charging higher import duties for video cameras, but if it records less than 30 minutes it's not classed as a video camera. Yeah, I know.

The DSLR will probably cope better with low light due to its larger sensor, and it will give you a shallower depth of field, meaning that you can get a blurry background and sharp foreground, which has a more cinematic feel. Of course if you want the background to be sharp this may not be such a good thing, and it also means you will also have to be more careful about focus.

If you're filming from a long distance you might want to see which camera has the better zoom. At least with the t51 you can buy a longer lens.

Audio on that camera should be fine, you might want to make sure the level is set to manual, otherwise it will turn the volume up everytime the speaker pauses.

As for settings, that depends on how you want to shoot. You could set it to full auto and let it drive, or you could go full manual so that you're in control, or you could use something in between. The aperture and ISO etc. will depend on the light levels in the room, and how much depth of field you want.