I voted for the Curve. Mainly for the fact that, it uses a standard 3.5mm Headphone jack, so if you're like me with a sweet pair of Skullcandy's (or other good headphones), you can use that for movies/music.
It also has a camera, which a lot of other Blackberry models don't have. Like most smartphones, it has expandable memory, again for the purpose of media (pics, movies, music, and applications).
Not to mention the Blackberry OS's interface kills Windows Mobile 6 by a long-shot.
Now, Windows Mobile is definitely useful, but it can get quit sluggish at times. I found myself constantly ending apps using the "Task Manager" so that it would free up system RAM and make the phone run quicker, as well as not drain the battery so much.
I've had my S720 replaced 3 times, and I ended up canceling my contract with them, and went Pay-As-You-Go, for the time being.