Two pipes compared to one pipe = better air flow.
It's all about the air flow Manufacturers releasing new exhausts generally all do duals now, having said that the differences on an N/A car are negligible anyway.
Price/exclusivity/sound are the usual factors with most customers.
I was the first supplier to sell the Invidia Gemini systems in the UK and sold dozens without issue so as biscuit says they are pretty good , I won't be beaten on price so if I can help drop me a pm. Plenty of others to choose from on the website below to Forum discounts on most systems
http://tarmacsportz.co.uk/350z-exhaust-systems.aspx