Shay Given is a better GK than Foster. Foster is a good shot stopper, but Shay is probably the best around at that. Foster can't kick, or command an area. The only benefit to Foster is that he is younger, but that would be reflected in his fee. Given has also shown he can handle the pressure of a big club, something Foster bottled at United.
15 Clean sheets in 44 games for Man City during the 09/10 season says all you need to know about Given.
