I've had the 8100 Pearl, the 8300 Curve, the 8310 Curve, the 8800, the 9000 Bold, the 9700 Bold, the iPhone 3G and the iPhone 3G S. I've also had lots of other phones for a while like the Xperia X10 (Android) and Windows Mobile phones but really the only two worth going for are iPhone and Blackberry. The android is truly awful. Amazing spec on the handset but poor screen responsiveness, poor call quality and too complicated to use.
Basically the phone you get depends on what you're comfortable with. The iPhone is easy to use right out of the box, has a fantastic selection of genuinely useful apps, and a FAR better internet experience.
Battery though, not as good as the blackberry, and also I find that when roaming, the blackberry is significantly cheaper to use (for data). Also good on the blackberry is the 'feeling' of reliability and durability. I don't think it's any more durable than an iPhone but they do tend to feel more robust. Stay away from the Storm though, you will regret it. Why buy a blackberry that doesn't have the key feature of the blackberry - a comfortable keyboard?
Now, as regards Keyboard, the Bold 9000 has the best keyboard on any phone ever, hands down. The 8800 is a close second.
The keyboard on the Curves is quite loud when you're trying to text discretely and also it's very small. It's still very good, and back when the three options were 8100, 8300 and 8800 it was the Curve (8300) that I went for because it offered the best balance between features, size and usability.
The Pearl has suretype which drove me mad, but is actually quite easy to get used to if you give it a day or two.
But the iPhone.
The keyboard is genuinely FANTASTIC. It's accurate, responsive, really big buttons, has auto-correct (that you can tell not to as well), like it's a joy to use, and then the best thing is that it goes away when you don't need it so you can use the whole screen for the best internet experience, for bigger videos, etc.
You will at first, find the physical keyboard of the blackberry reassuring and helpful, you know where you are with it, but with the iPhone, you realize that you don't need physical buttons and a tangible click to know you've typed the right thing, you will get used to it and you'll find it a joy to just give a feather-light touch of the screen rather than the effort of pressing a button.
So decision time. and a few last factors!
Cost! Go for what's on special offer! You will be happy with either. If a blackberry is £300 and an iPhone's free, get an iPhone. If an iPhone's £200 but on £45 a month and the blackberry is £200 on £15 a month, get a blackberry.
Travelling: What's your priority? Staying in touch (email / msn mad?) or finding your way around.
The iPhone has unrivalled local apps for when you're travelling. Interactive Tube maps for example, but if you want to stay in touch and there's not free WiFi everywhere, data roaming is horrifically expensive. On the Blackberry, it works out a lot cheaper to leave data roaming turned on than it does on the iPhone and you can stay in touch quite easily that way.
I carry both the 3G S iphone and the Blackberry Bold 9700 and am REALLY happy with this combo. iPhone great for music, videos, web when I have WiFi, tube maps, travel guides, camera, etc... and Blackberry great for emailing, msn, uploading photos to facebook, tweeting, etc.
Hope that's a bit helpful. Happy to answer any questions!