The K750i and the W800 are exactly the same electronics with a different case and different firmware. The 'extra memory' issue is just that it comes with a larger MemoryStick Duo card in the box. Oh, and a different headset, but again that's something you can purchase as an accessory.

You can actually flash a K750i with the firmware from a W800, if you know what you're doing (for example, if you want the better media player from the W800 but don't like the vile orange/cream styling).

Another vote for the K750i from here.