Good comparison...both are good phones that if I wanted to, I'd get both. The SLVR is is slim and looks gorgeous. However, it does have the 100 song limit. But I'd push for the W810i because it has built-in radio and a flashlight. And since I have a Vaio, it's easier just to plug in the memory card instead of carrying a card reader most of the time. Also, I have seen other W810i owners change the housing with that of the W800 & K750i. In any case, you'd be getting a great phone.
But yeah, like Ace said, you'll probably get more responses that are not biased towards Moto's.