Speaking as someone who owns a Windows Phone and who had an iPhone, let me sum it up:
If you're of the mentality of a 14 year old, and like shiny things and "apps" that are little more than web based time wasters and don't really use your phone for work, you'll LOVE the iPhone, especially when daddy pays for it.
If you actually USE your phone for work, get a Windows or Blackberry phone.
The iPhone JUST got the ability to send picture messages....2 YEARS after it was released! The Moto RAZR could do that...in 2001.
You can only run one app at a time on an iPhone. A windows or blackberry phone allows you to switch apps at hand and run text messages, email programs, browse the web and run office apps all at the same time, and switching between them is pretty damned easy.
Also, the iPhone has yet to come out with a decent editor for windows office documents. Yet, windows mobile comes with an entire mobile office suite! And, with most 6.1 and nearly all 6.5 windows mobile phones, full-web browsing is automatic now

And, with 6.5 you really don't have to compress any data files because they've improved the media player. Soon, I hear they may be pushing forth an upgrade to the Zune media player for their phones, and let me tell you: THAT would make a windows phone very nice.
Another thing is the camera: The iPhone uses a 1.3MP camera. To put that in perspective: The new HTC's coming out on almost every carrier have at LEAST a 3 to 5mp camera. One or two even have 8 to 10mp!!
And, the iPhone doesn't have removable memory, so you gotta use iTunes EVERY TIME to get stuff on or off your phone. And, it doesn't transfer files via bluetooth. Oh, and forget getting an extended life battery, because you cannot remove the battery on an iPhone!!!
There is also a HUGE explosion of phones coming out (about 12 this year!) with the Google "Android" OS. I've heard it's pretty good, and has alot of the same features as Windows Mobile, but I have yet to use it.
But, do your research online. Check out tech blogs like engadget.com and cnet.com for the newest of the new phones coming out. Because alot of times the cell phone stores only carry half of what the company REALLY offers in the store. So, you have to do your research because the perfect phone for you may have to be ordered, but it's soooo worth it to get the right device.