This is probably one of those questions where there really is no right or wrong answer. I guess an automatic would be more "drivable" if the driver were not comfortable with a manual transmission. Alot of high-performance Z owners prefer the feel of shifting for themselves, and controlling the engine's revs and powerband. Guys that are going more for straight-line performance will usually choose an auto for the faster shifts. I think generally you'll find an automatic setup may be a little lighter and more compact than a 5- or 6-speed setup. Finally, an automatic MAY be a little more gentle on your drivetrain (driveshaft, u-joints, rear differential, half-shafts, etc) than a manual, but that's purely personal opinion in my part.
Do some searching in the "Drivetrain" forum, and you'll probably find more info than you could possibly hope for! Good luck and happy motoring.