When I give a talk (e.g. in a department seminar) it is more convenient for me to sit down, since my body is more relaxed and it is easier for me to concentrate in the talk.
But, I have been told that sitting down makes me seem less serious, and it is more professional to stand up while talking.
So, is it better to sit or stand?
It depends on what you wish. If you wish to present yourself, stand and walk. If you wish to convey material, but keep yourself in the background, sit.