Auto is the way to go. They hold way more power in stock form.
??? I guess completely stock manual vs. completely stock auto, the auto will hold a little more, but once you upgrade the clutch, you have a solid tranny that should be able handle at least 800 HP. An auto that would handle that much HP would cost way more than that (I haven't priced them, but I am guesing at least 3x more).
I am not saying the OP should get a stick. There are definitely trade offs (mainly drag racing, boosted launches, speed of shifting), but if you aren't worried about acceleration, the stick does have some major advantages (much cheaper to make reliable 700+ HP, towing, and complete control of the drivetain/no unexpected shifts). Some people just like sticks, some just like autos...it just depends on your preferences as to which would suit you best.