Comparing two texts by switching between two windows is not convenient. To do it quickly and easily, simply use Differences Examiner.
As its name suggests, Differences Examiner is for comparing and analyzing two texts on a Mac. It can compare and analyze any kind of text, for example, a plain text or a source code. For that, you can either drag and drop or copy and paste the texts you would like to compare on the window. Its GUI allows you to work in good conditions.