One word - pinch!
On the seriouse side...I never heard of Tilt, just saw now that it is a Windows device. The most important feature of both of these is that they are
platforms.
The iPhone is a smart phone. It is a platform for apps to be delivered, as is WM. That means that the iPhone (and Windows mobile) can and will be expanded over time. You should look at today's features and capabilities but also look down the road to the future. Which device/platform do you see growing? Which has an OS that you enjoy to use? Which has the developer base that you look to to develop apps that you need?
The simple break down would be Windows Mobile is more for corporate users and iPhone for consumers. But each is adding features on the other's side of that fence.
Here is a break down of your feature set today:
email, messaging, internet(3G so i can quickly browse at school), etc.
iPhone: has all these minus the 3G. Mail needs improvements, hopefully coming in 1.5/2.0, which I expect in late Feb with SDK. SMS now has multiple recps in 1.3. Safari is excellent. I find EDGE ok in some spots, bad in others. Wifi helps there.
watch tv on the go
Is this some 3G streaming? I'd rather sync exactly what I want to watch. With iPhone you have iTunes stuff and your own content in MPEG4. It's a great media player (maybe the best out there?)
stream radio/xm radio
To each his own but again, I'd rather program my own media content. Radio (even XM) is dead to me.
two navigation systems (telenav and tomtom)
GPS
iPhone: you may win here. 1.3 added Locate function, which is nice. finding stores, restaurants in Maps is great on iPhone. But no turn by turn GPS.
3MP video
iPhone: 2mp stills, a bit lacking. Do you really use this though? 3rd party proof of concept has shown video app on iPhone.
microsoft office mobile
That is a nice feature, if reliable. Again hopefully coming via 3rd party app. You can use some of the web 2.0 online office suites on iPhone though. Files2phone looks cool:
http://www.apple.com/webapps/productivity/files2phonesandmore.html
Zoho Office:
http://www.apple.com/webapps/productivity/zohoofficesuite.html
a more detailed weather system(radars, text forecasts, etc)
There are a bunch of web apps for this.
http://www.apple.com/webapps/weather/
and a nice large keyboard.
iPhone: keyboard that guess what you are saying! And it's only there when you need it. To me that is huge.
You should wait until the SDK comes out, which I'm hoping also brings us another big update to 1.5/2.0. I think the iPhone is going to continue to amaze. YMMV.