Sublimerge automatically integrates with your version control history, and lets you compare between revisions, branches, remotes, and the staging area.
Sublimerge can only compare entire file diffs, but not two selections within a file. Comparing within files can be useful for example, by refactoring two similar functions to use a shared function. With Sublimerge, you need to copy the sections into two new temporary tabs and compare between the two. This can be cumbersome, as if you have another untitled file, you won't be able to know which one is which.
Instead of creating a new tab with the diff in it, you can tell FileDiffs to open the diff in an external diff tool for side by side comparison and other features.
FileDiffs allows you to compare any arbitrary text through multiple commands:
Compare the current file or selection with any other file via a file select menu
Compare the current file or selection file with previously selected tab or window or panel
Compare the current file or selection with your clipboard
Compare between two selections through Sublime Text's multi-select feature
In addition to providing shortcut commands for custom shortcuts, FileDiffs adds new command pallet entries. If you don't use diffs often enough to warrant memorizing a new shortcut command, the command pallet provides a quick and easy way to access the plugin.
After running FileDiffs, it creates a new diff file in a new tab, which doesn't have the benefit of showing the diffs in context. However, it is possible to open the diff in an external diff tool instead of creating a new tab.
Diffs are only computed when you call it via key bindings, so if you edit the file after generating a diff, the old diff will still be there until you clear it or regenerate it, which can be kinda confusing.
The only way to compute a diff is through key bindings, and the default key bindings are ctrl+k, ctrl+d for diff, and ctrl+k, ctrl+c for clear diff, which will require some memorization or customization.