Sorry got to disagree with you, had both too, X5 I only had for 3 weeks mind where as the 535d I had for 12 months and 25k miles, the X5 feels roomier inside, but the E61 touring has more usable space, the boot can take a lot more. In fact the usable space on the X5 was the only thing I found disapointing.
And the mpg is around 10% better round town and 20% better sat at 90mph, not worth worrying about I agree, but still slightly better, and the 535d is a much more fun car.
Cool , i loved both cars