As you know, everything is std on the 06 4.8is. So, a fair comparison from a pricepoint persepective would be approx. $72K for a new 07 4.8.
I've seen 06 4.8's on eBAY, etc for around $57K.
I ordered an 07 because it's slightly bigger, had the new gadgetry, etc. I also couldn't stand the black on black (clean freak).
Most importantly, I could afford it. If $'s are an issue, and you don't have kids (I like the 3rd row/space) grab the 06. Peformance wise, differences are minimal.
2006 X5 4.8is - SOLD
2005 530i - SOLD
2003 330 - SOLD
Ordered 4/25: 08 550 - Space Gray, Blk int, Bamboo w/Sport, Cold & Nav.