Having owned two 4.4i E53's and currently a 4.8i E70 I can say they are both equal to me wrt driving experience. The E70 has more power then the 4.4i's I've owned (2000 and 2001) but it weighs more than the E53. Later model E53's along with the 4.6 and 4.8 are probably faster than the E70. The E70 also has more gadgets than the E53 which makes it more creature comfortable (for those who like those things).

With that said I have a special fondness for the E53. I can't explain why...probably because it was the first X5 I ever owned and I couldn't believe how such a large vehicle could handle.