The first and most important question is do you need to collaborate on your word processing documents with other windows users? If you do then you really have to get Word. Yes Pages can import and export Word Documents, and for simple documents this can be ok, but you can run into conversion problems with longer, more complicated documents.
Word vrs Pages
Word and Pages are good word processing applications. Both applications are just as easy and capable to create, edit and do basic formatting to text. Pages has a superior user interface that is simpler and easier to use.
Both Pages and Word provide great looking templates to get you started. Word does provide more templates than Pages, however there are a number of websites where you can obtain additional templates if you need them (free and paid).
Although Pages has fewer templates than Word, unlike Word all of the templates have multiple layout styles for different pages, all based on the same design theme. For example in the Design Newsletter template page styles include cover, 3 column with slider, table and 3 column etc.
Word 2011 also contains themes – customisable selections of fonts, colors, and paragraph formatting, which can be applied to multiple document types to make different documents visually consistent. Pages does not have an equivalent function.
When it comes to inserting and manipulating images and other objects such as tables, Pages manages this better than Word does, however both applications give you access to the Mac Media Browser. The Mac Media Browser gives you easy access to your photos in iPhoto or Aperture, music in iTunes and movies in iMovie.
Both Word and Pages allow you to add charts and tables into your documents. These can be created from data held in spreadsheets.
Word is better at handling long documents with extensive footnotes and endnote capability (you have to choose one or the other in Pages) , complex Table of Content handling including automatic updating and Bookmarks. Pages also lacks a citation manager, which can be found in Word.
Summary
Word has more business oriented templates and the themes function can be used to give all your documents a consistent look, and perhaps has the shortest learning curve if you familiar with Word for Windows.
Pages is quicker, has a better user interface and simpler to use , all of the templates have multiple page styles and manages photo’s and other graphic elements more easily than Word.
Pages is probably best suited to shorter documents, particularly those with high graphic content.
Word is more suitable for longer, more complex documents that require the additional functionality like advanced Table of Content and footnote/endnote management.
However I do not believe that you can buy Word by itself, it has to be purchased as part of Office for Mac.
Other Options
If you just want to focus on writing you might want to take a look at ByWord or iA Writer, these are text editors, rather than full featured Word Processors, but both allow you to use the built in OS X Spell and Grammar checkers. These applications allow you to focus on your writing in a full screen, distraction free environment.
Or if you plan to write long documents like a thesis or book then take a look at Scrivener.