Now we get to the meat of the debate: big screen smartphones need big screens. That's why these phones are so big, but which has the best display?
The Sony phone has an IPS LCD capacitive touchscreen, 16M colours. It's a 5.2in display with a resolution of 1080 x 1920 pixels. That gives a pixel density of 424 ppi - not up there with the best QuadHD displays, but impressive nonetheless. Sony also offers its solidly impressive display technology including Triluminos and X-Reality (intuitive contrast enhancement).
In the case of the iPhone 6 we have 4.7in display. It is an LED-backlit IPS LCD Lenovo VIBE X2, capacitive touchscreen with 16M colours. You get, as you would expect, shatter proof glass with an oleophobic coating. Into this display is packed 750 x 1334 pixels, making for a pixel density of 326ppi. This is very much an iPhone screen: sharp and colourful, with realistic colour reproduction and good viewing angles.
Both good displays, but put them side-by-side and we have to say we prefer the Sony's. Whether that makes it worth purchasing over the iPhone 6 is another matter. (See also: iPhone 5s v Galaxy S5 comparison review.)
The Xperia Z3 comes with a quad-core Qualcomm Snapdragon 801 processor - no 64-bit here in preparation for Android L. It is clocked at 2.5GHz and paired with 3GB of RAM. The iPhone 6 comes with an Apple A8 processor - a dual-core chip running at 1.4 GHz. And it has only 1GB RAM. So the Xperia is measurably more powerful than the iPhone, but in our use we found them both super fast handsets, and the iPhone's benchmark scores are excellent. It is possible that if you run multiple high-power processes the Sony will be more capable. (It's also possible that its battery will run out sooner. We haven't yet tested to see.)
We've discussed onboard storage options before, but it is important to point out that the Sony offers external storage in the shape of MicroSD card slot that can take up to 128GB. Apple doesn't like expandable storage because it may affect performance, but I'd much prefer the Sony's take on this subject.
Both handsets offer 802.11ac Wi-Fi and Bluetooth 4.0, and both are LTE 4G phones. The Xperia Z3 has a non-removable Li-Ion 3100 mAh battery, the iPhone 6 a non-removable Li-Po 1810 mAh battery.
The iPhone 6 sports an 8Mp rear-facing main camera that can capture video up to 1080p at 60fps, and 720p at 240fps. The Xperia Z3 by contrast offers a 20.7 Mp camera that captures video up to 2160p at 30fps, 1080p at 60fps, and 720p at 120fps. It's also an HDR video camera.
Around the front both boths have webcams, the iPhone's being a 1.2Mp camera and the Sony's a 2.2Mp. You cannot fully judge a camera by its specs, but Sony's do tend to be especially good.
Android vs iOS is a conundrum. Android isn't like it used to be: if you are new to the smartphone game there's no obvious winner. These are the two most popular and best mobile operating systems around so it's about picking which one is right for you.
In essence, if you are a long-term iOS user you are probably best off sticking with what you know. You have after all almost certainly spent a lot of cash on apps that you'll have to spend again in Android. But it is worth considering that your iTunes music files will work in Android, and Android offers the opportunity of shopping around for music, movies, books and TV shows. See also: Just another opinion about Apple's new iPhones.
We're Macworld, and we love Apple products. But even we accept that in the smartphone world Apple's rivals are catching up. In the case of this rival, however, we can't see many compelling reasons to choose the Xperia Z3 over the iPhone 6. Cameras, perhaps. Display, definitely. But although the Xperia is more robust than the iPhone it isn't as pretty, and in all other respects - including price - we'd choose the iPhone 6.
The iPhone 6 starts at ￡539 inc VAT for the 16GB model. You will likely need more space than that, and the 64GB will set you back ￡619, ￡699 gets you a 128GB iPhone 6. The iPhone 6 has just gone on sale, and is widely available throughout the UK.
The Xperia Z3 is on sale at ￡549 inc VAT for the 16GB model. There is a 32GB model likely to be available in some territories.
But you certainly don't need to choose the Xperia Z3 simply because it is cheaper. It isn't. 
The Xperia Z3 utilises the classic styling of an Xperia smartphone: square shape, side mounted buttons and rounded sides. It continues the combination of glass and aluminium, with the introduction of nylon corners which should prevent damage in the occasion of dropping the phone.
Sony continues to offer a dust- and waterproof design so that means all ports apart from the headphone jack are underneath flaps. The phone is IP68 rated which is slightly higher on the dust side of things and can be submerged in 1.5m of water for up to 30 minutes. You'll notice that new colours are available beyond black and white so you'll be able to choose from copper or silver green too.
The new Xperia Z3 is 7.3 mm thick and weighs 152 g. However, at 72 x 146 mm lenovo P780 it's still a big phone. By contrast at 138.1 x 67 x 6.9 mm the iPhone 6 is certainly not a small phone, but it is uncommonly thin, and smaller than the Xperia Z3 in general. And 129 g is very light for a large-screen smartphone: much as you would expect from Apple. If you want a big-screen handset with a smaller chassis, you are going to choose the iPhone over the Sony.
The iPhone 6 is a very different device from the iPhones of the past four years. The new phones have rounded edges - not just the metal, but the edges of the glass front are curved. This and the overall thinness make it a very comfortable fit in the hand.
In general then we prefer the iPhone's build and design over the Xperia Z3's, but the Sony phone is indisputably more robust, as well as water- and dust-proof. You won't need a case for it in the way that you will the iPhone 6. (Related articles you might like: iPhone 6 review | iPhone 6 release date | iPhone 6 Plus release date | iPhone reviews.)