Amazon have put a pre-order price for £600 on their website... thats fucking crazy... they do state they have no idea what the price will be - I guess they have learnt their lesson from underpricing the Wii U before official prices were released
Going on the little we do know - I'd go for £349.99 for X1 at launch.. and £499.99 for PS4
Edit : Theres also been retail listing for Mirrors Edge 2 for the X1 on the Germany/Italian amazon sites - I really (unexpectedly) enjoyed the first one.. so that would be an awesome launch game - and quite possibly an exclusive too.
Edit, Edit : I've actually gone back on my word.. and pre-ordered both consoles at Zavvi for £399.99 for each one - the way I see it.. I've paid no money, I can cancel whichever one I want.. I dont have to worry about missing out by not pre-ordering early and if the price of each console is revealed to be quite a bit more.. I'll get a console on the cheap (Im sure the PS4 will cost more than £400)