I won't give you a reason because quite frankly I don't care what kind of phone
you have. Just get a 3Gs if you were liking your iPhone. Who cares what we say.
As the owner of both an iphone 3G and the Bold 9700 I would have to say that, as much as I LOVE the 9700, the screen will take some getting used to size-wise if you're used to the iphone's screen. (I tried the Storm 1 for the screen size for this reason, but that didn't go so well .) Apart from the screen size, the 9700 is amazing.
It really depends on what your needs and wants are... What made you leave the Curve for the iPhone in the first place? What were you not getting from the iPhone that is making you think about a BB again??