rogerxp,
This is a pretty good resource. http://370zexhaust.com/
Stillen and Tanabe are both round 4.5 inch tips. I think most are 4 inch rounds which actually seem to look smaller than stock because the stock tips are oval... for me this wasn't a look I was interested in. Bigger is better right?! Off the top of my head I think Akra are actually bigger (125mm) but it's only because of the outer carbon fibre or titanium casing bit on the tips. I may be wrong on this though. I think if you went for Akrapovic with the titanium tips that would look cool IMO because you would get larger metal looking tips but most people seem to get the black CF tips design but it's all a matter of preference, mate. I'm not sure when I was researching that I came across any of the better known brands that do oval tips. You could get a specialist to make them or choose a company that has interchangeable tips designs - I think magnaflow, scorpion and possibly blueflame might do this.
From what I've seen the Stillens stick out slightly more and are not cut to an angle which to me look a little more aftermarket but at the same time bloody nice... it was a close call between these two or Akra really.
Tanabe would sound lovely on a roadster I will guarantee you that! Do it.