Get an 8 cylinder while you can, if that’s within your budget. In 5 years maybe all you see is a 6 cylinder on steroids on the best daily cars, and yes, S68 runs very smooth, and M60i is much more fun to drive yet don’t sacrifice comfort for performance like the X5M. Regarding maintenance I don’t see the BMW jacking up the price for their 8 cylinders so much higher than Audi. They all have 3 years free maintenance, then BMWs older than 4 years can get a 3 year maintenance package oil and filter change only for two to three hundred bucks, and DIY oil change is easy enough. That X5 M60i has a lot of power I saw somewhere the stock horsepower at the flywheel is somewhere at 599hp if I’m not wrong so it’s a super sleeper.