I signed on with Cingular with a w600i because I liked the looks and unique-ness of it. 6 months later my friend's dog ate it, so they gave me the d807 as an insurance replacement.
Having lived with both of them on a daily basis, I'd choose the d807 hands down. As deviantmotoSE points out, the Sony is FAT. Even in my work khakis it created quite the "unsightly bulge". In jeans, forget it. Also, the Sony has to be "keylocked" when not in use, otherwise it will start browsing the web or playing your mp3s while in your pocket. There's a handy pull-switch on the side to do this, but it becomes tedious to do this after every call. The d807 is much quicker to slide open and get down to business. Plus, the camera on the d807 is better and the eye-candy in the menus is cooler.
If you're looking for true mp3 player though, the Sony is the one to get. It was built with this purpose in mind, unlike the d807. The media player is easy to use and the Sony comes with decent headphones. My d807 came with no headphones and I have to order them for $15 from Samsung. Despite the advantage of expandable memory in the Samsung, the actual media player software is pretty utilitarian.
I love my d807, despite it's few minor quirks. But then again, I have a 30GB iPod for my music listening duties. I only use the phone at the gym.