Both guns have strong and weak points. The Colt is the stronger of the two guns and can shoot any .357 Magnum ammunition you like in it, but Colt no longer makes DA revolvers so factory support and gunsmiths familiar with them is getting scarce. Also, the Trooper Mk. III should not be dry fired without snap caps as the firing pins are prone to breakage when dry fired and replacement is a factory-only job.
The M19, on the other hand, is such a popular revolver that, while no longer made, most repairs can be done by any competent gunsmith and parts are usually not problematic to find. The drawback to the M19 is that it's not very tolerant of full-power magnums with lightweight (<140gr) bullets. Lightweight magnums are prone to cause erosion and eventually cracking of the forcing cone in S&W K-Frames and S&W's stock of K-Frame magnum barrels ran out long ago. If you stick to .38 Specials or .357 Magnums with 140gr or heavier bullets, however, a M19 will likely give you several lifetimes of reliable service.
As to the prices, both revolvers would be high in my area unless they had a particularly uncommon barrel length, finish, or other feature. That being said, prices of used guns can vary substantially from one area to another and I do not know what said guns would normally go for in your area.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar