The same could be said the other way round.
Having seen a lot more of Sorensen this season than Foster (obviously), I can think of one big mistake that Tommy has made resulting in a goal (Boro away), and can think of two for Foster just off the top of my head (Robinson's goal for Spurs, Vassell's goal for Man City). If I can think of those, I'm fairly sure there must be more.
Every goalkeeper makes mistakes.
This is true and Sorensons mistakes are not why I would rate Foster above him. My reasons for doing so are based on aerial command, organisational ability and age. Sorenson is generally as good as most other top keepers in the world at the moment (including Foster) at shot stopping, he severely lacks in the other departments.
Personally, goal keeper is not a priority position to be replaed for me but if we are going to do it, Foster would be a good choice.