Simon I have both the 17 inch Dell XPS and a Macbook Pro 17 inch..
After using PC's and most Dells for over 20 years 2 years ago I got my first Macbook Pro.
In a nutshell I will never buy another Windows based computer again. The difference in usability, functionality, stress free operation is just worlds apart.
With windows machines I was always tuning, fixing, updating, modifying, tweaking, repairing, upgrading,
With Mac machines i am simply totally productive doing what I need, want and like to do.
They are also georgeous design works of art.
I have not regretted switching platforms for one minute. Just make sure you get something decent to begin with.