Fiddler is the winner every time when comparing to Charles.
The "customize rules" feature of fiddler is unparalleled in any http debugger. The ability to write code to manipulate http requests and responses on-the-fly is invaluable to me and the work I do in web development.
There are so many features to fiddler that charles just does not have, and likely won't ever have. Fiddler is light-years ahead.
