The iPhone 6 features a 4.7 inch, 1334 x 750 (326 ppi) "Retina HD" display, while the 6 Plus has a 5.5 inch Full HD 1920 x 1080 (410 ppi) "Retina HD" display.
Both the Nokia Icon and HTC One M8 5 inch full HD 1920 x 1080 (441 ppi) display (however, not the same quality displays). The 1520 showcases a 6 inch Full HD 1920 x 1080 display featuring Nokia's ClearBlack technology.
The iPhone 6, weighing in at 4.55 ounces, is 5.44 x 2.64 x 0.27 inches. The Plus model comes in at 6.22 x 3.06 x 0.28 inches, with a weight of 6.07 ounces.
The One M8 is 5.76 x 2.78 x 0.37 inches and weighs in at 5.64 ounces. The Icon is 5.39 x 2.80 x 0.39 inches and weighs 5.89 ounces. The 1520 is 6.41 x 3.36 x 0.34 inches and weighs 7.27 ounces.
The iPhone 6 features a 8 megapixel camera with Apple's new 'Autofocus with Focus Pixels' aimed at faster focusing, allowing you to take nice pictures even faster.
The 1520 and Icon boast a 20 megapixel camera with Nokia's PureView technology and ZEISS optics. The One M8 has a Duo Camera with 4 megapixel UltraPixel technology.
The M8 comes packed with a quad-core Snapdragon 801 clocked at 2.3 GHz. The Icon and 1520 are both powered by a quad-core Snapdragon 800 clocked at 2.2 GHz.
The iPhone comes with an A8 64-bit processor and an M8 motion coprocessor. Apple, again, does not provide universal specs, just showcases statistics promising that the phone is 'faster.' (Which is not necessarily a bad thing, as looking at the 2.3 GHz v. 2.2 GHz probably make some people go towards the M8 even if there is no perceivable difference).
So ... ?
The above information is not significant by itself. It will decide which phone to get. It doesn't even contain all the specs. However, looking at these specs I find the iPhone and WP devices to be on even ground on a hardware level.
The iPhone 6 is a bit smaller, slimmer, and lighter than the One M8 and the Icon. The iPhone 6 Plus is smaller, slimmer, and lighter than the 1520. But that was not surprising, as Apple always attempts to slim their phone down as much as possible. However, the WP devices are close enough that you have to hold one to know how you feel about it.
The screens are well matched, and each has been praised by reviewers. The pictures you will be taking, however, have some parity. The One M8 performance has been reported as under whelming, while the Lumia devices are in a class of their own. The iPhone has long been praised for its pictures, and it continues with the current model. It will depend on your preferences and use cases to decide for yourself.
The silicon powering these devices is more than sufficient for each - especially as both Operating Systems are optimized and not very resource-hungry.
These specs form, in my opinion, the backbone of a phone. The screen, dimensions, camera, and CPU are the foundation the rest of the experience is built - and remains dependent - on. Looking at these specs independently of the rest of the phones' features, I find these 5 devices to be well matched.