This really depends what your going to use the car for...
I've owned a 370Z, its fun, its beautiful, if you get it loaded it has every feature you need. However, if you want power it is the weakest out of these 3 in terms of tuneability per $. However, if its a sporty ride you want, the 370z is the best drive of the 3. When i cross shopped between a 370z and a 335i i couldnt help but come away feeling the 335 is designed for a more mature crowd. After driving the 370z, the 335 felt numb, heavy, sloppy, the seats werent that good and well, not like a sports car. However it did have alot of torque. When i got back into the 370, i was relieved. Again it depends what you like.
In terms of interior quality, i feel like a loaded 370z (like i had) is better than the bmw interior, with more features.
Now if its power and reliability you want. The mustang is the ideal choice. You would need every bolt on under the sun and Meth to keep up with bolt on and tuned 5.0 in the 335i, but you wont have the reliability, or the sound, or the handling.. Yes, the mustang does handle better than the bmw, so dont argue.. On the down side, the interior isn't exactly up to par compared to the other two without the premium package, and the seats, well you'll be using your left knee often in tight corners.
In conclusion... If you care what people think, get the bmw. If you want a car that will move like a raped ape with a sound that will give your bros, broners, get the 5.0 .
And if you plan to track the car extensively, and dont have too many friends... get the 370z.