Excel's handling of multiple open windows has long varied depending on the version you're running, the OS you're using and the options you have set. Excel 2013 eliminates most of the variability: every spreadsheet in Excel 2013 opens in a separate instance of the program.
Excel's full adoption of the multiple document instance (MDI) model means that if you have two Excel spreadsheets open on Windows, you can switch between them using Alt-Tab. That was possible in earlier Excel releases, but only if you tweaked various options and settings.
This approach you can use the Windows snap option to easily compare two screens; use the Windows-Left and Windows-Right options and you can easily compare two spreadsheets side by side. The downside is that you can't disable the option if you'd prefer all Excel Windows in a single instance. I'm actually happier having them separate; what do you think?