Xara Web Designer is intended for web graphics, web page and website design and it has web features and templates that are not in Photo & Graphic Designer. Photo & Graphic Designer is intended more for print graphics, illustration and photo work - it is suitable for creating individual web graphics but it does not have all the web features of Web Designer - in particular in relation to the creation of complete web page and websites.
Xara Designer Pro X v8 is our top of the range title and has all the features of both Web Designer MX Premium v8 and Photo & Graphic Designer MX v2013, plus other pro features (such as support for Pantone and color seperations).
So if web design is your main or only interest, then Web Designer is the best option. If you have a wider graphics interest then you might consider buying both Photo & Graphic Designer and Web Designer, or purchase Designer Pro.
Note: April 2013. We have just released Web Designer 9 Premium. The new features and templates of v9 are not yet in Designer Pro, they will be added to Designer Pro X9 when it is released.