Don’t know much about the Nikon. There is a lot of anticipation around the 5D being updated this year. I bought the 40D in preference to the current 5D – for that reason as well as the newer technology in and performance of the 40D (simialr to the new 1D in many ways)
I am a Canon user but if you if I did not have a heap of Canon glass I would go with the Nikon D300 over the 5D unless you must have a full frame sensor. The Canon 40D is a superb camera and costs less than the Nikon but the D300 is better. I use a 40D as backup for a 1D MkIII. They are both almost identical in image quality. The only thing better about the 1D MkIII is a faster shooting rate and tank like build quality.