I've used blackberries, iphones (original, 3g and 4) and now the D2G. I left the iphone 4 months ago because of AT&T reception issues in San Francisco. In my opinion, the iphone 4 on verizon is the best available cell phone on the market.
While I like the physical keyboard on the D2G, it's no faster than the soft keyboard on the iphone. As far as hardware, the screen res on the iphone is higher and the camera has better low-light performance. Differences in chip speed have little effect on daily use, in my opinion. I experience lags with my D2G that make it feel slower (though this may be attributable to using Fission over stock moto)
The real difference is software. The corporate email solutions (including moto's, stock android and touchdown) are not as good as iOS's. This includes big stumbling blocks (like AOSP's limitation on 30 days of synced email) to little things (like the handling of automatic address completion). The music player is weaker than iOS - including both interface controls and syncing issues. For me, these are the two common uses of my phone, and I've found that moving to the D2G made both experiences worse. That said, switching to verizon has made my voice/data connection much better.
Right now, I think that iOS is a more polished and complete mobile os. Android may be catching up, but if I were in your position, I'd swap for the iphone. Given that my 30 day window is long passed, I'm waiting to see what apple releases this summer.