Mac – Fixed tab-spacing for TextEdit on Mac

I think this answer here is what you're looking for:

How does Mountain Lion set the default text editor for the "open -t" terminal command?

To change default text editor to e.g. textmate, execute this command in terminal then restart:

    defaults write com.apple.LaunchServices LSHandlers -array-add 
'{LSHandlerContentType=public.plain-text;LSHandlerRoleAll=com.macromates.textmate;}'

There's a ugly way of doing it, as far as I know.

Once the file has been edited just try to save it. TextEdit will prompt you saying something like:

The document "Untitled.txt" couldn't be saved. Other application modified the file.

Giving you the choice to either Restore or Save anyway.

While Save anyway will keep your changes and save as a new version the changes made by the other application, Restore will update your file with these new changes.

Unless there's a specific reason to use TextEdit I'd recommend using any other text editor, such as TextMate, which supports the automatic refresh.