Yep I've had both although not both working at the same time because I stupidly fell on my K750i. The keypad is better on the K750i, I prefer the camera cover and with the exception of the music button I can find nothing better on the W800i. I also prefer the black of the K750i and you probably won't be able to use the music button on the side as there isn't one on the K750i so it will be a bit useless.
If you like the look of the W800i then fair enough but I don't think much of it, the only reason I got it was that it had a 512mb Memory Stick as standard and it would have cost me about same getting a replacement screen and a new Memory Stick as it did to buy the W800i.