According to what I've been able to dig up on the Six, it's 33 ounces for the 4" barrel versus 40 ounces on the 4" GP. That's a significant weight difference as a carry pistol goes. The GP is definitely a beefier and heavier built gun with quite a different feel than the Sixes. I own the GP, but having been considering a Six as a possible carry gun or trail gun, rather than the SP.
North Country Gal pretty much nailed it.
I put my 4" and 6" sixes on the scale and they were 33 oz and 37 oz. I don't have a GP, but according to the Ruger site they are 40 ounce 4" and 45 ounces 6".
The GPs have a heavier top-end with their full lug barrel, and the Sixes have heavier back end with their full grip frame. This leads me to believe that most of the weight difference is due to the larger cylinder and frame on the GP.
Agree with North Country Gal. The GP 100 and Security Six are great revolvers and I like both. To me, the trigger feels a little better on the GP 100. For extended carry, I'd vote for the Security Six.
If you compare like models, there is virtually no weight difference between a GP-100 and a Security-Six. My RDA-H (Sec-Six heavy barrel) has the same exact barrel profile as my KGP-141 (4" GP-100 w/full shroud barrel ) and weighs just a half ounce more than the GP when the grips are removed. GP-100 grips are a couple ounces heavier than the small Sec-Six wood grips. Service-Sixes, Speed-Sixes, and standard barreled Security-Sixes are a few ounces lighter. This makes virtually no difference in "carry weight" or felt recoil.
When Ruger designed the GP-100, they went from a full sized grip frame on the Sec-Six to a grip stud on the GP. This allowed them to make the GP-100's cylinder frame, top strap, and cylinder more massive without increasing the overall weight. The diameter of a GP-100s cylinder is a bit larger.
If you are familiar with S&W revolvers, the Security-Six is to a K-frame S&W what a GP-100 is to a L-frame S&W. For a "carry gun" the only notable difference between the two is the GP-100's larger cylinder is a bit more evident.
When you mechanically examine both revolvers closely, the GP-100 has a far better design in all respects, however a Security-Six is still a mighty fine gun. Of the two, I prefer a GP-100 because it is a current production gun with many more options for grips and sights. The Security-Six went out of production in 1988 so factory parts or aftermarket parts are getting very hard to find.
The difference between one of my fixed sight 3" or 4" GP100's vs. one of my Speed or Service Sixes, as far as weight and size, is negligible. My 4" DAO fixed sight GP100 fits in an IWB holster sized for a 4" Service Six and I don't feel the difference when carrying.