Distinctly different animals.
FX is a nice crossover. It has a little of everything. However it will never handle as well as a 5 series, even one without the sport package.
Everyone always hears about the horror stories of BMW ownership. I have had an E28, E30, E36/8, E85, 3-E46s, and maybe something here or there I forgot. My mother has had an E30, 4-E46s, E39, and E92. We had problems with only 2 cars out of all of those, and the problem consisted of a faulty DSC DME in one and a faulty alarm in the other.
Want ultimate driving bliss go with the Bimmer. Want cheap services and about 8.5/10th the driving thrill of the BMW go with the Infiniti.