Smith
Of the two guns, the S&W was a reaction to the COLT and a better design. S&W started having so many problems with the K-frame .357 magnums, they brought out the 581 and 681 fixed sight guns and the adjustable sighted 586 and 686. These guns are built on the L-frame specifically for the .357 magnum cartridge and are quite strong.
COLT had the advantage with the Official Police mid size frame. It was sized for the old .41 COLT round and was slightly larger than the SMITH. COLT replaced the midsize guns (except for the PYTHON) with the MK III models, which were cheaper to make, Then they started getting complaints about timing problems, if I remember correctly.
COLT eventually brought out the MK V series, but then revolvers were on the way out with cops.
Jim