Datsun/Nissan made several 4 and 5-speed manual transmissions throughout the 13 year reign of the original Zcar. Although there was a european 5-speed available for the 240Z, it was rarely seen in the US. It wasn't until 1977 that a 5-speed was added in the US. The 1977-80 5-speed was consodered a "wide-ratio", the 1981-83 a "close ratio" transmission. Close ratio transmissons are preferred for road racing as they allow the driver to maintain a tighter rev range between gears compared to wide ratio versions.

I wouldn't lose sleep over which is the "optimum" trans for your car, for most people the "best" one is what you have in your car right now. If you decide to upgrade your 240/260/280 4-speed to a 3.90 or higher rear end you should upgrade to a 5-speed for the additional gear.