You can compare two variables in a bash script with an if block like so:

if [ "$EDITOR" = "$PREFERRED_EDITOR" ]; then
  # do something ...
fi

If those variables are equal, then the contents of the if block will be executed.

Notice that both variables are wrapped in quotes. This is to avoid a potential syntax error. If the quotes were excluded and one of the variables happened to be unset, then the comparison would evaluate to:

if [ "vim" = ]; then
  # do something ...
fi

That would cause an error, rather than evaluating to false and moving in. Wrapping each in quotes allows an unset variable to turn into an empty string ("").