It's all depends on your personal preference, how much money you have, how much money your willing to spend and whether or not you can find one.
Your going to spend alot to make either car go fast, but the GSX will be cheaper....as for reliablity......I would stick with the Nissan. I have a friend with a turbocharged (up the a$$) GSX and he drove a SR powered S14. All I can say is he actually liked the S14 better than his.....and for some reason he know work at a Nissan dealership.
Anywho...both good cars....but I have to go for ANY S14 (95-98 240SX)....mainly because I have one, I like the styling.....and you don't see too many on the streets. Also I want to have a fast AND sexy Japanese coupe.