I've had a '04 3.0L for about 100K miles now - some around town use, but most of it has been long distance highway. That 6cyl has given me plenty of hp on the interstates, and boost when I've needed it (ask the WV state boy who warned me @ 90mph last month). Check around the various BMW forums and you'll find the M54 6cyl engine gets pretty good long term reviews in both the E46 and E39 models as well - except for coolant system components (i.e. expansion tank, etc) - but those same problems afflict the V8 as well.

Fewer repairs and lower maintenance costs for the 6cyl. More HP for the 8 - your choice.

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2021 X3 M40i Glacier Silver
2016 X5 xDrive 35i Mineral Silver

Gone but not forgotten
2017 X4 M40i Black Metallic
1985 325e Cosmos Blue
2006 330i Monaco Blue
2006 X5 Ti Ag
2000 528i Ti Ag
1986 535is Black (great ride)
1985 528e Polaris Silver (GTA in NYC)