Ultra steady.. lol ya right I only have the use of my right arm and im a diabetic which means even on a good day my hand still shakes...lol
In this case you might want to think about the very considerable difference in size and weight between the Canon f4 and the Sigma f2.8. For many people it's not a problem, but for me (I have arthritis), it was. I used to have the Sigma non-OS f2.8, and while I liked the images I got from it, I ended up selling it mostly because of the weight; I found I was leaving it at home more often than not, just so I wouldn't have to carry it around. I replaced it with the Canon f4 IS, and couldn't be happier.
If you haven't actually handled the two lenses, I'd strongly recommend doing so before making your final decision. Outdoors in daylight, the f4 will do a fine job for you and will certainly be much easier to hand-hold for long periods. If you don't actually need the f2.8, the Canon is a great choice.