I noticed that many people prefer to do audio only sessions for language exchange. For me, both (video and audio) is ok, so I'll just go with whatever the other person prefers. I did sometimes do sessions where I'm on video and the other person is on audio but I prefer to do that, when I'm in the teacher role. I think teachers should be willing to show their face, especially when they talk to beginners, because mouth movements and facial expressions give important clues for learners. Often I start sessions with video, when I'm the student, so the teacher and I can greet and smile at each other and then we switch to audio only for having a better internet connection. For language exchange it feels a bit odd to be on video while the other person isn't.
What are your preferences?
Oh, I much, much, much prefer having video. One of my teachers was absolutely brilliant at hinting at corrections by slightly raising her eyebrow. And with a language companion, when you get stuck trying to explain something, it is so helpful to be able to wave your hands, or act out, or grab objects from your desk or point your camera around the room.
One of my community tutors has a lousy Internet connection, and it is often necessary for us to turn off our cameras in order to improve the sound. I regard that as a necessary evil. I really don't like it.
Personally, I prefer Video calling just because I like eye contact and also I use a lot of hand gestures, body language, and nonverbal communication to get my point across. Especially When I am teaching an elementary or beginner student. I mostly have to nod a lot because the student needs assurance that I am understanding what he/she is saying. My emotions come out in the form of facial expression. If there is frown on my face and I look perplexed, my students will start again and correct himself without my interruption. Also, I use whatever object I have on my study table to make a point. But some students like only voice call and that's fine by me as well.