I really like both the iPhone and Curve. I tend to use the Curve more, mainly as it now sports my work number on it. I also prefer the fact it is smaller and much lighter than the iPhone, I suppose the physical keyboard on it also adds to the appeal. Aside from the Treo 680 and iPhone it is the best smartphone I've used to date. But in actuality it is not so much the form factor that attracts so much but ultimately the choice between the Curve and iPhone rests on the available software. There are some really good BlackBerry applications, I love SafeWallet Pro, Bloomberg Finances (also available for the iPhone but much slower to load on GPRS), push Gmail and Calendar syncing, Berry Weather is great, TwitterBerry is good plus Repligo is one of my all time favourite mobile apps. In addition, the camera on the Curve is rather good.
Meanwhile, the iPhone has far better RSS software, is far far better for using eReader and reading eBooks, has better media capabilities and has better, more quirky type software that is dead cheap to buy. It also has WordPop and some great games. If I could merge the two in terms of software I'd have the perfect device for my needs which, incidentally, appear to becoming more basic with each passing month.
I really can't decide between the two. I suppose the Curve is more of a business device but it is also very capable of handling most media stuff with the possible exception of RSS. I would say it is the best combination of powerful business features with consumer centric meda attributes out of the box, better than Palm and Windows Moble anyway in my view and considerably ahead of Nokia. Some of the native Nokia software for handling media is disappointingly dated looking and basic, not to mention fiddly to use and get the hang of.
If someone took the Curve away I'd be happy to use the iPhone and if someone took the iPhone away I'd be similarly happy to battle away with the Curve. I'm looking forward to but also dreading in a way the arrival of the Palm Pre as it may well bring in a 3rd top class option. Either way, Symbian and Windows Mobile devices will never again darken my doorstop again until they learn to raise the bar and catch up with their competitors instead of living in some odd Sinclair C5 inspired world.