Fiddler has a nice feature for comparing two –and only two- sessions. This feature comes in really handy when your sessions become complex and hard to compare.

For example, let’s say you have 2 sessions and you need to compare the 2 sessions character by character. 

Comparing Sessions in Fiddler  

Now to compare them, select the desired sessions and right-click on any of the selected ones and click Compare (CTRL+W).

This will launch a WinDiff window to show the comparison.

 Comparing Sessions in Fiddler

Note: if you got an error message like below, then you need to download WinDiff tool.
And then in the Main Menu go to Tools > Fiddler Options > go to Tools tab > browse to the actual WinDiff tool by clicking the three ellipses button. 

 Comparing Sessions in Fiddler

Comparing Sessions in Fiddler