I believe that the W810i is just the same phone has the k750i, but flashed with better firmware (better media player?). k750i also has a lens cover, w810i doesn't. With some of the earlier k750i's, you can flash them with W810i firmware.
W810i comes with a 512MB memory stick; K750i only has a 64MB stick i think.
k750i can be picked up brand new on Vodaphone for £80 delivered.
W810i is £50-70 more. (£150 on vodaphone)
I've been looking at k750i or w810i, and on some shops it looks like they are discontinuing the W810i.
K750i, you get a better deal, but if you want to buy a memory stick then it's +£20 ish, so w810i is £50 more for a later phone that looks better. Possibly get the w810i cheaper though.