These are close to each other in so many ways, I could go with either weapon. In actuality, I have several GP100 sixguns, in various barrel configurations, and an S&W Model 19, plus a Ruger Speed Six. Notably, the original, pre-Hogue, GP100 factory grip seems to have been designed by my long-lost twin, and the GP100's trigger is also shaped perfectly for my index finger, so I favor the GP100 for those reasons, even before such considerations as longevity and potential toughness are considered.
An S&W K/L-frame has to be accessorized a bit to work well for me.
With either brand, be sure to cherry-pick among several for a smooth trigger. A smooth trigger gets smoother, even if a bit heavy. A gritty trigger is likely to require some 'smithing to correct.