Windows and Linux web hosting from Daily.co.uk are both feature-rich, reliable services with a variety of packages to suit your needs. Choosing between the two operating systems really comes down to what technologies you want to use to build your website.
Windows is highly recommended if you or your developer will be working with Windows-specific technologies like ASP and MS SQL, whereas Linux is the ideal environment for PHP, Perl and MySQL. In general, if you don't specifically need Windows-only technologies then Linux is likely to be the best, most flexible choice.