I bought an HTC Desire HD having been an iPhone 4 user for the previous ~6 months. I wouldn't describe myself as an iOS or Android fanatic and I therefore feel able to present a detached analysis of how the Desire HD compares to the iPhone 4. I should note that I have been a previous owner of an iPhone 3GS and iPhone 3G. I'll not compare the HTC to those other than to say that it is definitely superior. The iPhone 4 presents a more worthy and relevant competitor.
OPERATING SYSTEM:
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The principal benefit of the Apple iOS is that it 'just works'. Very few issues are encountered with regards to application incompatibilities or application developers being forced to cosider and cope with various hardware specifications when writing applications. On the iPhone 4 you can be sure that if you download an app from the app store, it WILL work on your device. In addition, the operating system, menues, folders and general interface are in my opinion definitely more user friendly. However, the level of control you have over the functionality of your device is vastly better under Android than it is under iOS. My opinion is that Android is more suited to technically profficient users. That's not to say that luddites everywhere wouldn't be able to use an Android handset, far from it, but for those who don't know a kilobyte from a megahertz, iOS presents a simpler and more easily navigated user interface. Being a tech head myself, I greatly appreciate the vastly superior level of control I get with the HTC under Android than I had on my iPhone 4 under iOS 4.3.
The number of applications available for the iPhone series of devices is at the time of writing greater than the number of apps on offer in the Android marketplace. However, Android's market share seems to be increasing inexorably and it's probably only a matter of time before the number of apps available for android equals and then surpases the number available for i-devices. There are a number of developers who seem reluctant to develop apps for Android purely because the iPhone is seen as being a more glamorous and fashionable item to own. For example, Stratfor (who provide eo-strategic intelligence reports) and my damned bank both have iPhone apps but no equivalent for Android. Whether this will change in the future remains to be seen, but for those who are desperately fashion conscious (I for one am not) then the iPhone may be the way to go.
SCREEN:
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It has to be said that the Retina Display on the iPhone 4 is glorious. It really does offer an incredible viewing experience with ultra sharp graphics and superb levels of contrast. Coming from the Retina display (3.5" diagonal) to the larger, lower ressolution display on the HTC Desire HD, I was concerned that I would really mis the resolution of the iPhone display and that this would spoil my user experience. It did not. The 4.3" (diagonal) display on the Desire HD is a gloriously large screen on which all sorts of goodness can be displayed. Colours are sharp and vibrant and the screen provides very good levels of brightness and contrast even when viewed in direct sunlight (the Desire HD like the iPhone 4 has an ambient light sensor and dynamically adjusts screen brightness to compensate for ambient illumination). Though the difference in resolution is just about noticable if you look very closely, the screen on the Desire HD is itslef a very high quality display and I very quickly forgot that it isn't quite as high resolution as the retina display on my iphone 4 was. Indeed, I'm loving the sheer size of the screen on the Desire HD.
The only strange behaviour I noted with the screen on the Desire HD is that the amount of adjustment that the automatic brightness system makes changed over the first few days that I used it. At first it seemed to be reluctant to make large alterations but now it's happy to change the screen brightness to whatever it needs to be. Perhaps the auto-brightness control has some sort of heuristic learning function.
ERGONOMICS:
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Both the Desire HD and the iPhone 4 are fairly weighty items, however this provides both with a tactile feeling of quality. The machined aluminium unibody contrustion of the Desire HD is probably a bit more robust than the gorilla glass panels on the iPhone 4, but given that both phones are very expensive items I would very much recommend something like the Otterbox Commuter case which offers superb levels of protection while still offering access to all the controls.
One criticism I have of the Desire HD is that the on/off button and rocker volume button have virtually no travel and provide very poor tactile feedback. Additionally the volume rocker button is very long and sits in just the wrong place, making it very easy to annoyingly reduce the volume of a call when picking up the phone. The iPhone 4 by contrast has near perfect controls. This is a small annoyance but for pure functional ergonomics the iPhone 4 is slightly superior. The battery cover on the Desire HD is also a pain to remove. However, having a replacement battery (and removable memory card for that matter) are features that the iPhone 4 just doesn't offer.
PHONE & MUSIC:
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Both the Desire HD and iPhone 4 provide superb sound quality when making voice calls. The iPhone 4's issue with reception is by now very well publicised though putting a case on the iPhone 4 solves this issue. Contacts and call data is handled differently but equally well on both handsets.
For the audiophiles amongst us, the sound quality on the iPhone 4 and the desire HD are both superb and both probably equal. I certainly can't detect and discernable difference. The only advantage that the iPhone 4 has over the Desire HD is that the iPod application that comes with the iPhone is vastly superior to the defauls music player on the Desire HD in terms of ease of use and functionality. However, there are a very large number of alternative music player apps on the Android marketplace so if you don't like a particular app that comes with the phone then it's simple to download a different one. And this is a general strength of Android over the iOS - Android is vastly more configurable and customisable than iOS.
CAMERA:
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For those who love taking heaps of photos or lengthy videos, the Desie HD and the iPhone 4 offer 8MP and 5MP camera sensors respectively. Both offer the ability to record 720p HD video at 30 FPS. As others have commented, the quality of the photographs produced by the iPhone camera is probably slightly higher than the camera on the Desire HD, but the latter offers many many more options for video and still photography. Both the Apple App Store and the Android Market offer many, many photo editing applications.
CONCLUSIONS:
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To be sure the HTC Desire HD and the iPhone 4 are both superb phones. For the non-tech-savvy user the iPhone probably offers a more simplistic and streamlined user interface. For the more tech-savvy user the Android operating system offers vastly greater control and customisation of your handset. The actual hardware inside the iPhone 4 and the Desire HD is almost identical, though the CPU of the iPhone runs slightly slower which offers slightly lower performance and longer battery life.
You may be wondering why I moved from the iPhone 4 to the Desire HD. I was thoroughy fed up with the heavily curated, dictated iOS environment. I want to be able to have a user-controlled experience on a top-quality phone and for me the Desire HD fulfils that very nicely. I'd never suggest the Desire HD as a realistic prospect for my 70 year old mother to use, but personally I love the freedom that it offers.It's also a superb phone on it's own merrits.