f4.5 IS will allow 1/15@4.5 (e.g)to get correct exposure while f2.8 can use 1/45. if the subj is a stone....quite alright. if the subj is a child...think 2.8 has a better bet under same lighting condition.
then if f2.8 IS vs F2.8 non IS....f2.8 IS has better change at least motion blur restricted more to subj moving than you handshake.
