if you're just going to install the axle-back's (mufflers only) - either one you will be fine with interior noise levels. However, if you plan on upgrading the ypipe, mid-pipes AND mufflers, then you will get into the area of louder noise levels.
I believe that Stillen makes cat-back systems specifically for the M35 & M45 - due to power/noise balance. In other words, they make an exhaust for a V6 which may not be compatible to a V8 and vice/versa.
In my opinion, if you are just looking for a little more noise when you 'step on it', then it might be an idea just to get the Megan mid pipe while keeping your stock ypipe and stock mufflers (just change the tips to larger ones and change the inlet pipe of the stock mufflers to 2.5" - to match up with the Megan mid pipes)
I currently have the following set up: Megan Verson2 Ypipe, Megan mid pipes, stock mufflers w/larger tips and larger inlet piping to mufflers (about an 8" section of piping that my exhaust guy welded on to replace the smaller stock inlet muffler piping) this set up, compared to many exhaust set ups that I've done on my M, it is quiet during cruising and no cabin drone...NONE. but, when I 'get on it', I have the benefit of better exhaust flow - performance, and better/sporty sound - it closely resembles the G35 stock exhaust but a little deeper. Now, I can be stealthy with the cops driving around since my set up looks stock. it also gets a lot of other Infiniti drivers by surprise "if" we do a little bit of spirited driving...I always get asked if my car is stock or what modifications I have
