OK so I guess it's swings and roundabouts. If you are planning on keeping the car for a good few yearsthen go for economy. If you are changing it sooner then why not treat yourself? ;-)
I had an X6 40d from new for just under 2 years, and now have an X6 M50d.
I wanted an M car but like the higher driving position of the SUV so M5 was out (M3 too small for everyday but want one for weekend garage weapon!). Love the sound of the X6M and the mahoosive bhp under your foot, but did not like the thought of 10mpg on an average day of lead footedness. The 50i does not fare much better if you check owners' forums.
Also liked the idea of having that 3rd turbo for lag free driving through the entire rev range; plus that ridiculously large amount of torque available from the diesel engine at low rpm. Check out Youtube clips for the nice engine note above 3k rpm also.
I do like the way the torque is fed in, on a continuous wave which does not relent. When you shift the stick to the left into S mode and floor it, it is like a greyhound out of the trap.
In Manual change mode using the paddle shifters you can shift gear later and enjoy it even more (if you have fingers about 3 feet long to reach the paddles that is!!)
My wife has a 2011 model X5 M40d, which is no slouch - but the 50d doesn't just pull away from it, it annihilates it. Yes, I feel it worth the premium over the 40d, because it ticks another box for me - having a car with an M developed engine (yeah yeah yeah I know it's not a 'proper' M car LOLOL).
I had to buy one the second I saw one in the flesh. I drove past it in a cab one day, called into the dealer the next day and ordered one. 6 months later and finally I'm in it