Basically, it comes down to your driving needs.. I REALLY like to drag-race..
I think auto-x is more fun, but far more expensive, with tires/brakes/wreaks being more likely...
But, back to the diff between the two, under the two styles of driving...
1st.. Drag-race.. If you put an automatic, and a manual against each other, the manual "should" be faster up untill traction, or turbo lag becomes too much of an issue... Basically on a 700RWHP car, the automatic will (Almost) always win. It can apply the power better, for a strong launch, with little or no wheel-spin. Plus, you keep your foot planted for the entire 1/4 mile (or more..) and have continuous pull... With the manual tranny, you will have to have a very fine balance between reving it high enough to get turbo spool, Applying the clutch withought spinning too much, or bogging it down, and killing your 60' times.. You shift 4 times and have NO forward acceleration while shifting.. No matter how fast you can shift, you cannot beat a built automatic.
2nd.. Auto-x.. (Or canyon carving, etc, etc..) This becomes more of a personal prefference type of thing.. A built automatic with lock-up converter, and GOOD tranny cooler should do fine in an auto-x/road-race situation, but doesn't give you the exact controll of gears that the manual would...
3rd.. Street.. Again.. personal preff.. I know that when I had a manual with a race clutch, I'd hate a cross-town drive with about 50 stops on the way, because your clutch-leg gets tired.. An auto is nice, because you just move along.. no muss, no fuss, but then again.. no real manual shifting fun.. I like both for the different reasons, but I'll have an auto for drag-race from now on.
Disclaimer: the 700 rwhp mentioned above means that the auto will have about 50-75 MORE HP at the crank, because auto trannies are power hungry...
On slicks.. your RWHP numbers would have to be higher for the auto to stand out as much, but on street tires, the auto should be faster even at the 550 HP level..
Hope that helps more.. I personally LOVE a 6-speed in the mountains at about 120mph..