Well, I have a bunch of SIG's - including a 226ST with the conventional trigger and a 229R-DAK with the DAK system.
How do they compare? The "conventional" SA/DA system has two modes of operation. A long, heavy double action pull and a short, light, crisp SA pull. The DA is typically about 10 pounds - the single about 4.5 pounds.
The advantage is that many people shoot very well, quite easily with the light SA pull. The downside is that many people shoot poorly with the longer, heavier DA pull. You also have to learn two different trigger pulls for one gun.
Neither of these is an issue with practice. After a few thousand dry-fires I can shoot better in DA than SA.
The SIG triggers are typically of very good quality in SA and DA - smooth with little overtravel or creep.
The DAK system is a twist to the "DAO" style gun. Typical DAO SIG's have the same 10 pound pull, all the time, as the SA/DA SIG's when pulling DA. The DAK system is also DAO, but is only about 6.5 pounds or so. It is also even smoother and more uniform than the already exceptional SIG triggers.
The downside of the DAK system is that is has two trigger reset points. A short reset that results in a slightly heavier (but still lighter than standard DAO) and the "regular" reset that results in a longer but lighter pull.
I really don't notice the different resets. To be honest, I am not really sure even which one I hit most of the time.
The DAK system is definitely superior to conventional DAO SIG's in my opinion.
I believe the DAK system was designed primarily to satisfy government and law enforcement requirements for DAO guns, but in a package that allows for easier and more accurate shooting than what many were capable of with the heavier DAO triggers.
I think SIG scored a winner with the DAK. I really like it. I have a bunch of SIG's and only one is DAK - and it just may be my favorite. Another plus of DAK is the hammer is totally bobbed (you cannot thumb cock a DAK) which makes for a more snag-free carry package.
One note on the SA/DA guns - by swapping out with a slightly lighter hammer spring you can reduce the DA pull weight some. If you go too far, you'll get light primer strikes (no go bang) so you have to be careful. Wolf sells kits with a variety of weights for a few bucks so you can experiment.
I would not mess with the DAK springs. I cannot imagine it getting much lighter than it is, reliably.
I am not an expert. I would also suggest SIG forum for some detailed technical discussions of the DAK system, if you are interested.