Replying to Importance of Image Stabilization

Just curious, how necessary is Image Stabilization in a lense (16-35mm) when filming skiing with a glide cam?

I am debating the pros and cons of Canon's 16-35 f/4 IS, and the 16-35 f/2.8 II (non-IS), and I need some help.

I'm a huge fan of the 2.8 maximum aperture for low light situations, but if Image Stabilization is a big concern when it comes to video, I might have to go with the f/4.

Any first-hand experience or opinions are greatly appreciated!