Which is better and why?

haha I like both

I would say BMW X5, I love it. I drive it every day.
Denali is huge :)

depends on what ya want. do you want lots of leg room or do you want a more sporty vehicle? if you want the luxury of being able to go off road and actually afford to fix it if something breaks go with the denali. the bmw has the luxury but the 4WD is for snow sand and ice. the Denali is good for all of those plus mud dirt and slick rock.