The IS is weather sealed on a 1D. And if your working in low-light you get a stop of handholdablitity or so. I'd say if you can afford the IS f/4, get the f/2.8 non-is. 
If you can't afford that then get the Sigma f/2.8, if you can't get or don't want the potential risk of odd models etc. then get the f4 non-is.
If I were you stick with the f/4 non-is. Then if you getting into it get yourself a body that AF's really well in the future....
If you can't afford that then get the Sigma f/2.8, if you can't get or don't want the potential risk of odd models etc. then get the f4 non-is.
If I were you stick with the f/4 non-is. Then if you getting into it get yourself a body that AF's really well in the future....