Both cars sound awesome. I think I would lean towards the 2008 for the extra year of CPO and the HUD/paddles.
If you can, try to get the service history for each from a dealer and see if either car has been in multiple times for the same issue. That is a good indication that there is a problem that the dealer cannot fix (pass on that car).
The biggest concern would probably be a recurring top issue (see Top Not Locked threads on this board). Check for accidents reported on CarFax/CarProof and I would still get a pre-purchase inspection on the car you plan on buying.
Straight lines are for fast cars...turns are for fast drivers
'04 M3 - track
'06 650i Cab - daily driver
'02 Highlander - kid hauler
'01 330Ci - winter beater