Well, it all depends...
Blackberrys are slimmer nowadays, not clunky chunky like back in the middle ages, and it's really not just for the business users anymore. It's sleek, productive, sadly addictive, and fits virtually into any lifestyle.
A standard Blackberry has an organizer, calendar, email, phone (duh), map, media player, GPS, browser, wifi, camera (except the 8800 series), tethered modem, trackball, and it's a world phone. Both the 8800 series and the Curve have a qwerty keyboard, but the Pearl has the crammed version of it, which has 2 letters per key.
Windows Mobile...well let's just say, it depends on the user. Sure, half on the world's population lives on Windows XP or the problem-deal Vista, but the WM might not be for everyone.
The Pros of Windows Mobile:
If you're like me, if it has a touchscreen, it's a done deal. If you're not, they have non-touchscreens too. They have wi-fi, email, IM, and anything you want it to come equipped with. Think of it as your laptop with a smaller screen, less storage space, small keys, and basically fits into your pocket. Now, unlike the the Blackberry, Windows Mobile is basically an OS for phones. All it does is upgrade a cell phone to a "smart" phone. You can find it on phones made by brands from A-Z. Also has way more selection besides the candy bar style of the Blackberry.
Alright, so that's some of the good deets for the WM. What's the Con(s)?
Maybe it's just me, but the program is kind of slow, and in some areas it underacheived. For instance, the T-Mobile Wing. Great device and all, but sometimes mine just freezes up on me, and I can't run multiple programs at once because it slows down, and that drives me nuts. It might also have something to do with the fact that I keep dropping it, but put that into consideration too.
The bottom line is, they're both great and useful, but it all comes down to the user. I don't know your phone habits, but whatever it is I'm sure you know.
But if you really think about it, no matter how slow it is, I appreciate the fact that WM lets me edit and creat documents. I'm not an avid emailer, but I do write a lot, so that helps.
Anyhoo, hope that helped. If it didn't then, good luck on your smartphone quest.