The FX is a great value, it looks good (but styling is subjective), it is fast and handles very well. All this makes the FX a really tempting and well-rounded SUV. That being said, Nissan/Infiniti does a lot of cost-cutting on materials and things like that, in order to make the vehicle such a great value. Not to mention you do not get free maintenance for 4 years/48K miles like you do with the BMW.
They are both great SUV's and I don't think you can go wrong either way. Just keep one thing in mind, and I think Tom will agree with me on this one. When you go Infiniti (or any Japanese product), you lose that typical German character that all of us enjoy. And it's one of those things you don't really appreciate it until it's gone.
But at the same time, knowing you'll toast an X5 4.4i with an FX45 should make up for that character loss.