My opinion based post........

Manual is more fun
Manual is usually a bit faster (if it's an 833 trans that is) but NOT if it's a Kielser! They don't shift as Fast/Smooth as an 833.
Manual will get you better milage

Automatic is more consistant
Automatic is generally lower maintainance

If I were in your shoes I'd buy the Gear vendor unit regardless of if you run a 727 auto or an 833 4 speed if over drive is your goal, also look into Jamie Passons 4 speed OD unit, price is competitive and bullit proof..