wow, two very different cars you're looking at OP.
If you want a nice entry level luxury car, the is250 is the way to go. A bit more practical too and probably a bit better on mpg I'd imagine.
If you want a sports car, and all the good and bad that goes with that, then the 370z is the right car.
Test drive both, think about what meets your needs/wants best and then pick.
(I'd get the 370z, but I'm biased
)