Diamond: Stylus, handwriting recognition, graffiti mode, two cameras (front and back), programmable hardkeys, Windows Mobile (+relevant applications as the Sprite series), HSDPA, HSUPA, GPS, GPRS, 3G, TouchFlo 3D interface (which can be disabled and replaced by other UI's, if you wish), 4 hardkeys, approachsensitive multidirectional scrolling pad, acceleration sensor and far smaller. Plus, the backcover doesn't melt in your pocket nor does it develop tiny cracks over time.
The iPhone isn't bad by any means, but seeing the Diamond is slightly cheaper - at least in the Netherlands (491 euro for an iPhone (simlock-free) and 489 for a Diamond (simlock-free)) - I know which one I prefer and the only reason to go for the iPhone the larger memory bay or multitouch (and it has the Apple logo (L)!), while there are numerous reasons to go for the Diamond. They even figured a way to not need multitouch by allowing gestures like drawing a circle around what you want to zoom in on. Multitouch while on a bike is difficult enough.
Still, it sucks for those who bought the iPhone that the zirconium in the backcover of the iPhone turned out to be such a mistake. While I am a honest Apple fan, I think they lost this round. The Diamond is said to be iPhone killer #1, followed by the TouchWiz 3.
I agree there. I know at least six people now who have a Diamond and all of them are more than happy with it. These people are random people, heck, I met one in the cinema yesterday. The lack of a stylus for both opponents of the Diamond hurts badly in my eyes.
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