your request is very vague, the rx8 and 350z look very different, so unless you provide further information of what exactly you want is, i'll just give you an idea of your two listed.
if you do not plan to modify the car much, rx8 and 350z are both great choices for used cars and there aren't many more cars in that category, at least in the past decade
both are very reliable with their own small issues, but are in different classes. both are around the same price used.
rx8 is light weight and very agile, stock ones are very easy to drive and handles very well, gas mileage is terrible for the rx8 though, mainly because of the gear ratios to make up for lack of power and a high revving motor. good reliability if you keep it near stock with minor mods, but if any problems come up, bring it to rotary specialists, not mazda dealerships, unless rotary forums suggests specific mazda dealers, otherwise they are mostly your average mechanics who know nothing about rotaries and will most likely mess up your car even more.
350z is heavier by around 200 kg/440 lbs depending on the options, quicker in a straight line indefinitely, also very reliable, better gas mileage, easier to find service when required
out of the two, rx8 is more practical in terms of space if that matters as all to you.
i feel the 350z has a better designed interior, rx8 feels like cheap plastic all over, not to mention hinges easily get loose after short amount of time and start to making creaking noises while driving, and exterior seems to rust easier if you have snowy winters with salt everywhere.
i'd personally choose the 350z, definitely more bang for the buck since you are on a budget
saw your other post regarding rx8 engine with broken apex seal, which means it will require a rebuild or a engine swap. brand new rx8 engines from mazda are only around 2500$ if you give them your broken engine. installation differs from shop to shop, but at mazda, its around 1500-2000$. these are canadian prices, usually cheaper in the states.
stock rx8 motors easily go above 150 000 km, definitely shorter life than the 350z motor, and not as resistant to abuse, but then again, we are talking about abuse
rx8s should also be cheaper to buy used than 350z.
also, if you plan on boosting the car, definitely go for the 350z, boosting the rx8 will shorten the engine life significantly, and you won't see even close to the same power gains as to boosting the 350z
This post was edited by Zenius on Feb 27 2014 11:27pm