6 or 4 cylinder diesel?

[clowesy]

I'm currently helping my parents find a new car and after quite a few test drives they have decided on either a Mercedes E Class estate or a Jaguar XF Sportbrake. Whilst they like both cars in seemingly equal measure they aren't sure on which engine to go for in either. Due to my Dad loosing his job through illness a few years ago they were forced to sell their mk2 Focus RS and swapped to a more wallet friendly 1.0 Ecoboost Focus estate. With things now looking up they have decided to get a new car but want to keep the practicality of the estate but move a little more upmarket. They only cover approx. 15K miles per year and my Dad will use the car for business which will involve driving across the city a few times each day. In their current car he gets at best 35-38 mpg; not bumper to bumper traffic but plenty of traffic lights etc. Now, they like the 3.0 V6 diesels in both cars but are concerned they'll get far worse fuel economy than they currently get and wonder whether they should just settle for the 4 cylinder option. Has anyone any experience using either an E350 or XF 3.0d (240) for this kind of driving?

[clowesy]

Their current Focus is on PCP and due to end shortly. They don't want another.