I have both. The one benefit that the slide aside has over the lid bale is that when the lid is in the open position, it is nearly straight up and down vertical. That gives you good access to everything on the grate.
When the lid is in the open position with the lid bale, it isn't standing up completely straight. It still leans in towards the center of the grate at an angle. However, the lid bale has a much smoother action when opening and closing which I like. To me, the determining factor would be having to drill them holes in the kettle which I wouldn't have the heart to do especially if it was an older kettle. So for me, if it wasn't already equipped with a lid bale from the factory, I would go with a slide aside.