The luxury is identical between the X5 and X7. The X5 feels more athletic and overall faster in either trim (because it is). The X7 feels large and more top heavy but floats along like a deva. The front grill of the X7 looks fantastic. I adore the larger BMW grills as it looks more aggressive and fits the styling of the new platform. (Now the Q8 grill looks like a enormous black pig nose.)
It's all what you want. Both ride identical. That means smooth or sporting depending on the drive mode selected. The X5 is noticeably more razor sharp in handeling. You can't get away from the X7's mass.
If you "need" three rows then the X7 is the one. The optional 3rd row in the X5 is useless. If you don't need the 3rd row most all the time then the X7 is a lot of vehicle to live with. That's the trade off for such a large people hauler SUV.
I have a family of 4 and took them on a long trip with large luggage and never felt I needed any more room or space in the X5 50i. I own larger vehicles and they are a burden in the city or traveling when parking. X5 is optimum size for my needs even with money not a factor. A loaded up X5 50i M-Sport is $100k so the additional $10k for a identical X7 shouldn't be a factor at this price point of a luxury vehicle.
I demoed a fully loaded X7 50i with 2nd row captain. It was beautiful except one inexcusable sin. The black plastic 2nd row cup holders on the floor looked and felt ridiculously cheap. It was so out of character with everything else in the entire vehicle it made no senses.
It took up a unusual amount of floor space for just two cup holders that it looked like it was hiding something in the floor.
If I was getting a X7, it would have to be with the 2nd row bench to eliminate it. I think it's pretty accurate calling the X7 a stretched X5.