Are 2004-06 more reliable than <=2003

[Turbo_Bimmer]

Still collecting informations for my future car swap.

Is the 6spd tranny better than the 5spd? I also read that the 4.6 tranny was build with stronger parts. Does that make the 4.6 a longer living transmission?

Is the valvetronic reliable, or it is one more thing that can fail?

And finally, the transfer case. Right now, my 325xi 2002 has the old TC like the 1st generation X5. It is reliable, but, handling wise, I would like to experiment a X-drive as next vehicle. Clutches are wear items, and I don't think the X-drive is an exeption. Are there X-drive owners that had worn or failed clutch pack in their TC?.
Are there rebuild parts available in the rebuild market?
I read also the there is a bearing in the X-drive TC that is not available anywhere, true?

Thanks.