I have an HTC Mogul with a slide-out keyboard, and an ipod touch.
In my opinion, the full "landscape mode" qwerty keyboard of the HTC mogul soundly beats the apple soft keyboard, which soundly beats the miniscule qwerty keyboards you find on Moto Q or some Blackberries, which soundly beat any keyboard with multiple letters mapped to the same key.
The apple soft keyboard is much more useable in landscape mode, but curiously, very few iphone apps support landscape mode, other than Safari, even the standard "from Apple" apps. Meanwhile, virtually every Windows Mobile app, from MS or a third-party, supports seamless swapping between portrait and landscape mode.
Some of the differences are nearly "cultural". The apple encourages "type away, we'll fix it for you", which takes some getting used to. In particular, this encourages "type the whole damn thing, then go back and figure out what it got wrong". Unfortunately, I don't think the Apple interface is very easy to use for corrections, as compared to a full qwerty keyboard with cursor keys. Another problem is that this correction capability varies from app to app, some don't have it.
The apple encourages use without a stylus, so it responds well to "fat finger touches". This works well for gui buttons, scrolling, etc, but has an "imprecise" feeling when working with fine text. The Windows device expects a stylus, doesn't respond well to fat finger touching (you really need to tap with your fingernail), and really requires (and provides) cursor keys for navigation.
What apple did right is figure out that _THE_ big limitation on these devices is screen size, so they put a bigger and higher resolution screen, and came up with an elegant way of navigating around a screen. After using web browsing on the ipod touch, I don't want to use the browser on the Windows phone any more. The Apple made me realize how painful the Windows browser interface (both IE and Opera) are.
I do a fair amount of typing on my phone, and not just email/texting, but taking notes, and typing away at a command-line, and this is really the only area in which the HTC beats the Apple, but it's a pretty important one, unfortunately. Of course, I haven't really "tried my best" to find the best note composition app for the Apple, one that seamlessly switches between portrait and landscape, that supports auto-correct, that allows for zooming via multi-touch, one that doesn't try to use some cute hand-writing font, etc.
-harry