The Trooper/Lawman Mk III was a cost-cutting revolver that used sintered iron lockwork parts and they developed a reputation for brittleness and fracture. The Mk V series was a vast improvement and used cast steel lockwork parts that were more durable than sintered iron. The King Cobra was the final evolution and refinement of the Trooper Mk V.
Size wise, the Mk III is closer to the S&W L-frame, the Mk V is actually smaller, closer to the S&W K-frame.
None have a trigger action that really compares to that of the S&W K, L, or N-frames. The Colt Mk III and Mk V/King Cobra have coil mainsprings and shorter lock times than the S&W revolvers.
They are all handsome and generally accurate revolvers, but if I were to buy one, it would be either a Trooper Mk V or a King Cobra.