I own a Miata which has a 6spd in it and Ill tell you the gearing is far tighter and often ill find myself simply skipping over gears as they arent exactly needed. The 6th gear isnt a taller gear as you get with many 5 to 6 gear upgrades. The 6speed is slightly more robust, but unless you are planning on pushing 250+ hp through it, you wont see any gains there.
Many people who autoX and otherwise actually swap their 6 speed for a 5 speed.
