Lens suggestion for speakers at conferences

I photographed conferences for some time, and leaving the lens choice to one side - and I think you have made an investment in that already - I strongly recommend looking at a monopod.  It will offer three advantages:

1. It takes the weight of the camera and lens from you when you are sitting at the ready for perhaps hours on end, by reducing fatigue it is more likely you will get a steady shot and the camera is at the right height all the time.

2. It will vastly reduce the chances of camera shake.  There will be times when you want a narrow DoF, so a low f-stop, but there will be times when you want to have more in focus - perhaps for a group of people on the podium.  That is when you need to up the f-stop and accept a slower shutter speed.

3. If you decide to take video, the camera will not be inclined to jerk around so much, especially if you avoid the newbies error of moving too fast, producing motion sickness in your audience!

A monopod is more manoeuvrable than a tripod, and less conspicuous.

One last thought... rather than purchase any more gear, consider renting a good video camera on a tripod for a static position shot of the speaker and their presentation.  We did this for each speaker and many people were able to review the bits that they had missed.  Of course, speakers may well make their presentations available afterwards, but the static video captures the interaction between the speaker and their material.  If you are going to do that, make sure you have a good uni-directional microphone, or you will get the buzz from the audience.