I need some advice. I'm looking at purchasing one of the L series lenses. I narrowed it down between the Canon 70-200mm F/2.8 and the Canon 70-200mm F/4 IS. I was leaning towards the F/4 since it's a little cheaper and has the IS. I wasn't really sure if the difference in the speed of the lens would be missed. I would be using the lens mostly outdoors, but would use some for indoor use. I was hoping I could get some insight. Thank you.