I've been on Macs for over twenty years now, and have worked professionally on both, Mac OS and Windows machines.
Your question implies that you are concerned about the limitations (compatibility etc.) of a Mac vs a PC, and that you intend it for general use. If this is so, go for Mac Airbook. We have had it for over a year now, it's a wonderful and reliable machine.
There aren't any compatibility issues these days, Mac software allows exporting to cross-platform formats (PDF etc.) and to MS formats (Word etc). This is built-in, no additional expense. You can install Chrome and Firefox if you don't like Safari. And pre-installed Preview (it's an application, not just viewer) now has features on a par with Photoshop. I'd also recommend investing in iWork. With Lion, Apple started retailing its applications separately, each below 20 euros, just choose what you need most - Pages for word-processing and simple graphics, Numbers for spreadsheets and Keynote for presentations.
Recent scares about Mac viruses and malware weren't about Mac-specific viruses, but malware downloaded from cross-platform software, like Adobe. You have to be careful about security anyway.
There is also an issue of where you buy, not just what you buy. I'd recommend avoiding big shops, [mobile] phone retailers and certainly supermarkets. Mac or PC, you will need technical support, and no big retailer will have good apres ventes, as others here have said. Find a small to medium specialist shop that doubles up as a repair-maintenance business. It should be within reasonable driving distance. If you don't travel to UK on a regular basis, buy your main machine in France, and perifery/accessories on the internet or in supermarkets. When visiting the small retailer, find out if you can sign-up for telephone support and/or servicing on you premises, many do and it's good value when there is trouble.
As for 'build-to-last', I'm afraid it's a thing of the past. We need to think now in terms of 'last-for-the-while', meaning that machines become obsolete within a few years. Two years ago I discarded, with tears in my eyes, my 1991 LC, still working in OS 7, word processor and a few games. That's twenty years of use! When I serviced my 2008 iMac recently, the guy at EasyMac in Caen said, 'that's the last time, from next year they stop making spares for this model'.
So, when you do your costings, expect three-five years of use before you have to replace it again.
Hope this helps.