I think the greddy cost more. N1's aren't all that great. To me they are kinda cheap compared to how greddy makes their exhaust. I'm only saying all this cause my brother has a greddy SP. It a nice exhaust. The piping is nice, great for turbo applications. If you get apexi i would go for WS instead. Try tanabe. they arn't that bad either. All about the same, just the sound. The main thing you wanna look at is the piping diameter.
If you do decide to get a cat back, make sure you change the headers and cat. If you don't, you just wasted 500-600 dollars.