Other than the main factors like your budget, the quality of the video and sound, some other factors include what features you need for your webinar events. These may include the number of participants, if the platform allows for multiple presenters, if you'll be able to record presentations, and other customizations you may need.
It is also important that your webinar service provider allows you to test the webinar software so that your event will run smoothly. At EventX, you get a free demo so you'll know exactly what you're signing up for.