Rugerman:
I flat out suck at descriptive technical writing, but the simplest way to describe a striker fired pistol to a hammer/firing pin fired pistol is to compare a Mauser 98 to a Winchester 94.
When you pull the trigger on a Winchester 94 (or other traditional, exposed hammer levergun), the trigger releases the sear which drops the hammer which strikes the firing pin and drives it forward against spring resistance. Then the firing pin hits the primer and all kinds of magic takes place!
When you pull the trigger on a Mauser 98, the sear releases the spring tension holding the striker back, and the striker is driven forward by the spring. The striker hits the primer and all kinds of magic takes place!
Given not only the current Ruger issues with the SR9, but my general policy on new guns, I would choose the Sigma over the SR9 at this time. (First generation weapons are frequently plagued with issues the engineers and designers didn't encounter. Quite frequently, the second and later generations of such a gun can be absolutely frickin' stellar.
I am a "visionary conservative," or maybe a conservative visionary. All kinds of cool new stuff catches my eye . . . and generally I like to wait for a while and let them work all the bugs out (or, if they can't work the bugs out, quietly disappear from the market).
The Glock has proven itself a reliable and effective design, and so has the Sigma. It's kind of shocking to realize that the Sigma has been on the market for 14 years now. It does have a heavy trigger pull, but mine have always gone bang and put bullets where I was pointing them with boring regularity.
Given the plethora of other 9mms in the price range of the Sigma and SR9, my humble suggestion would be to consider other contenders.
Best of luck,