I wanted the W800i until I went in to an O2 store and played with them both. I am now the happy owner of a K750i.
Yes they are pretty much the same phone. The W800i has different MP3 player sotware and also has a airplane more where the phone part of the phone turns off and it's meant to be safe to use on a plane, but try to find a UK plane company that will allow this
They are the same phones hardware wise, despite some people arguing this on other forums. If you don't believe me then speak to Sony Ericsson support (not sales as they want to charge more for the W800i).
I thought the W800i looked better until I saw them together.
What tarrif are you going for? The O2 store I went in did me a £35 per month tarrif and gave me the K750i for free, so I spent my £30 on a 512 Meg memory stick.
The tarrif I am on gives me 1,000 any time any network minutes for the first 6 months and 200 messages. Then for the last 6 months I get 500 any time any network minutes and 100 messages.
And yes, it's a 12 month contract.
Aparantly that tarrif isn't available on the online store at present and is a limited offer with the K750i in store, shortly it will only be available for 3G and I mode phones.