ok, there are a lot of misconceptions about the supra and the 350z. most people think that the supra is faster but the z looks better. personally i prefer the look of the supra, but that is all opinion. as far as performance, the supra, if you get the 2jz-gte motor(which most supras dont have, and you either have to buy the motor seperate, pay a lot more than you would pay for a z to get a 2jz-gte, or just settle for the smaller motor) is an inline 3- liter. that motor was amazing for its time, but facing the facts, the automotive industry has moved on. the 350z also has an inline six, but it comes stock as a 3.5 liter with a six speed, no small motors or five speed transmissions are even made for the car. that being said, the supra has been around longer, meaning there is more available information for upgrading. unfortunately since the car is older and an expired model, new and quality upgrades are getting harder and harder to find so are waranties (which you'll want if you plan to upgrade). on top of the fact that the z has the same type of motor but bigger, the biggest advantage to the z is the availability of an AWD tranny, which would give the z a level of acceleration the supra could never dream of reaching.
which is cooler? i like them both, but the supra definitely has a legacy behind it that the z has not developed yet.
so, if your looking to buy one of these cars, heres what i suggest.
if you want:
a car to showoff at car shows/meets - supra
a car that wows people you pass - supra
a popular classic car - supra
a car for top speed- 350z
a car for drag or track racing - 350z
a fast car with out looking like a ricer - 350z
if you do decide on the supra, keep in mind that most cars its age have some maintenance issues, and the targa top models are notorious for leaking in the rain and ruining the interior.
hope i could be of help