Martin,
The sensor in both the 5D and 5D Mark II are good - canon hasn't fluffed a sensor design in any of their DSLRs, so i'd be surprised if they put a foot wrong with the latest camera. The bodies seem to be basically identical, with the same autofocus. The difference comes down to pixel density, and the trade-off between low light performance and resolution.
If you don't need the resolution, the only other reason to get the 5D II is the movie feature, which if you are into these things will be a wonderful feature.
Personally i'm disappointed that the bodies feature the same autofocus as before, and the slow laggy mirror compared to the 1D series. If you don't need the movie feature, i'd seriously consider the 1Ds series second hand. Ask park cameras if they have any 1Ds Mark IIs available, and what sort of price they are asking for. The 16mp sensor is lovely, fine for ISO1600. The body is very solid, very nice confident autofocus, and the shutter is very snappy - makes you feel you've got the shot you were trying to catch.
Cesare