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The (GP100) design uses a separate hammer and trigger spring, while the Speed-Six uses a single spring for both. As a result, a better trigger pull can be had with the GP-100.
Not true, JOG. It was the Redhawk that incorporated the infamous do-all single spring for both the hammer and trigger, not the Speed-Six/Security-Six/Service-Six models.

Here's an overview of the Ruger DA revolvers: http://wadcutter.blogspot.com/2005/03/ruger-da-revolvers.html