I was just driving a 2011 X5 with the new Idrive and navigation and it is really nice. But I did not buy the M3 for its luxury features. I would not have cared if it was a stripper with cloth seats and no Idrive or navigation. The luxury junk on the 08 seems fine to me, though the newest and latest suck junk is almost always improved.
Were it me, I would justify any additional price you are willing to pay on things other than a better navigation or Idrive. Maybe lower miles, newer model year so better resale value (though they will all be deflating with the new M3), maybe DCT versus 6MT, or competition package versus stock wheels and shocks, etc. But that is just me. If the doodads are the most important thing for you, get the doodads you want.