Whttail,
There are actually some very significant differences between the two designs. Overall, the Speed-Six is a "first generation" design along with the Security and Service-Six, and the Redhawk. The GP-100 is "second generation" along with the SP-101 and the Super Redhawk (it would have been more accurate to have named the SRH the Super GP-100).
The GP-100 uses a "peg" grip frame, a better trigger design, and a better lock-up system. The 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.
As FreeMe mentioned the Speed-Six is a tidier package, so if that's what you're after the Speed-Six is a good route. Personally, I kinda choke looking at the GP-100's peg grip - it just ain't natural.
Neither can be called elegant, but they're both great revolvers. The GP-100 is probably the strongest mid-size revolver ever made.
