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DAK is a Double Action Only pistol that had a reduced weight pull.
DAO means the hammer resets after every shot. There is no single action. The DAK is a DAO with a lighter pull.
Most sigs are DA/SA. Which makes the first pull a heavy double action, than every follow up shot is the shorter and lighter single action pull.
A lot of people say they have a hard time getting used to the DA/SA. I on the other hand adapted to it very quickly and for me and IMO (take it what it is worth, nothing) it is a fantastic shooting mode. I am more accurate with it, I find the principal of the only safety being you and a slightly longer first trigger pull to be fool proof and won't ever cause a problem in any situation you get into.
Again, as with most things, I would suggest trying them both yourself at a range that rents firearms and going with what you shoot better with, and feel more comfortable using.
Edit: Example from Sigs own product specs shows that a DAK 226 has a 6.5 lb trigger pull every shot. A DA/SA 226 will have a 10 lb first pull, than every other round will have only a 4.5 lb pull.
One thing you can do that makes those DA/SA even easier to shoot with is have a little trigger work done and you can have the DA shot be closer to 6-7 pounds and than all the SA shots more around 2-3.