All the extensions have their advantages.
If you are based in the UK or have a mainly UK audience it is work looking at .co.uk as your site will come higher on searches in the UK.
If you have an international audience then .com or .net maybe better.
.net is not as popular as it once was, but you are more likely to find the name you want..
If you can't decide then you can register both as many domains as you like! Please note only the first domain is included in the HolidaySiteBuilder package.