You don't get bigger than the 27-inch 5K iMac, which boasts a screen resolution up to a mammoth 5120x2880. Some designers might still be a bit sniffy about the glossy display, but those in modern iMacs are vastly improved over previous generations, and can be successfully colour-calibrated if you're not happy with what you get out of the box.
Any of the existing models will do the job for print design. Pay more, you get processing power boosts and improved graphics cards, but the £1,449 model should be fine. For a more responsive system, you might consider swapping out the hard drive for an SSD, though. RAM can be upgraded at a later date.
Best Mac for web design: MacBook Pro with Retina display
Web design canvases now vary wildly (in the sense responsive sites must support anything from a watch face up to a massive telly), but so too do work environments. While it's undoubtedly advantageous for web designers to have a big display, it's even more important to be mobile, in order to take on new jobs at a moment's notice.
A MacBook Air would be sufficient for web design tasks, but if you've got the money, go for the MacBook Pro with Retina display. The high-res screen gives you a better impression of how your work will look on mobile devices and modern notebooks, a Pro isn't much heavier than an Air, and a Pro can be configured to be far more powerful. We'd recommend going for the mid-range 13-inch model (£1,199), which has a 256 GB SSD anything smaller is miserly. Consider increasing the RAM to 16 GB on purchase, too.
Best Mac for interface design: 27-inch iMac 5K
Interface design can of course encompass web design, but with this option we're thinking more in terms of apps. Many designers these days are creating interfaces for iPad, iPhone and Android apps, along with content for smart televisions and games consoles.
The ideal Mac here would give you a 1:1 impression of what you're designing, at full size. The 27-inch 5K iMac's 5120x2880 display just about manages that, even enabling you to squeeze in an iPad Pro interface in portrait (2048x2732). A cheaper alternative would be the 21.5-inch 4K iMac (from £1,199), which boasts a resolution of 4096x2304 enough for an iPad Air display in either orientation or an iPad Pro in landscape. With the 21.5-inch model, be mindful you cannot upgrade the RAM at a later date, so start with 16 GB.
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