Not so big a difference, really. There is a tiny bit of dimensional difference, but I don't think it's enough to proclaim one to be superior over the other in absolute terms.
FEATURE------COLT AGENT--------S&W MOD. 638 BODYGUARD
Cylinder dia.-----1.40 inches---------1.30 inches
Barrel length-----2.1 inches----------1.9 inches
Height-----------4 inches------------4 inches
Weight (empty)--14 oz.--------------14 oz.
Capacity---------6 shots-------------5 shots
The weight comparison is for alloy framed guns, but the other dimensions are the same as above for Colt D-frame and S&W J-frame steel snubbies. The Agent is about 1/4 inch longer and 1/10 inch thicker. Not much difference, but the Colt Detective Special and Cobra models do have longer grip frames than the Agent. The shorter grip frame is why I prefer the Agent.
In the early 1970's my first CCW was a vintage Colt Detective Special. It was compact enough for my needs, but it was heavy to carry and the hammer snagged on clothing. Eventually I sold it and bought a S&W 638, which I carried for over 20 years. I loved the 638 for its snag-free draw and easy carry but I never felt like I could hit with it as well as I should. It was and is a confidence issue.
I always wanted an older Colt Agent with the factory installed hammer shroud and finally found a nice used 2nd generation at a gun show last year. I put a Tyler T-grip on it but, other than that, it is completely stock. I can shoot 4-inch groups at 25 yards with this Agent. It's all I can do to keep all the shots on a piece of typing paper at 25 yards with the Bodyguard. Granted, 25 yards is kinda long for a snubbie, but if I practice at that distance I shoot better at the shorter distances when I have to.
So, for me the Colt works better and is what I now carry. However, since I don't have a bias for Colt against S&W (or vice versa) had I been able to shoot the Bodyguard better than the Agent then the Bodyguard would still be my carry weapon (after all, I carried it for over 20 years). I say use whatever gun works best for you. I think it is important to note that the dimensions on the two guns are so similar that I cannot feel or observe any real difference when either gun is on my hip or under my arm. I'm not a big guy either at 5' 10" and 160 lbs.
Just as I think the size differences are insignificant, I also don't think the 5 vs 6 rounds issue should be a show stopper. Accuracy and hit probability carry more weight with me than capacity.
Finally, the J-framed S&W's really took off in the '60's and '70's more as a reaction to price than specifications. My 1970 Gun digest shows S&W J-frames priced from $84-$90, depending on the model. Colt D-frames were priced from $104-$112. Even back then Colt was pricing themselves out of the market as a result of their labor-intensive design and Ruger was just getting started in the revolver market. Eventually Colt just gave up and left the small-frame snubbie market to their competitors. It's a shame, too. They had some nice revolvers.
-Bob