I have owned both. I like the BMW better for handling, but the infiniti is more luxurious; everthing on the bmw is an option, the infiniti is all standard.
I now have a G35 and my wife has an M35. There is little difference between them for the most part, other than size.
I would have bought another bmw but I didn't like the style that much. I had zero maintenance costs for 80K miles and was real happy, then when I went to the dealer it was ~$6K worth of work that needed to be done. Rather put that cash into a new car and ended up with the infiinti because the bmw dealer jerked me around.
Plus bmw had a design flaw in their window motors. I had 13 replaced in 5 years.