I think at the moment im swayed towards the 400. For me it seems like im going to be paying an extra £70 for a bigger jog and less features...
You can also look at it like this. I'm going to pay an extra 70 pounds for a professional jog, for better turntable emulation and a professional feeling.
If you prefer efx and/or like the digital storage media compatibility above turntable feeling you must choose the CDJ-400.
I hope you can see the difference. The CDJ-800 is meant for old skool dj-ing, while the 400 caters the younger generation dj's, who like producer's features i.e. a load of efx and digital compatibility.
Probably the technology behind a pro grade jog is more expensive.