There's basically two variants of the old three-screw: the "flat-top" is the earliest and commands a high price, while later ones have the same two "ridges" alongside the rear sight body that the New Models have.
Three-screw non-flatties aren't all that rare/valuable, and are basically smaller than the New Models. The Old Model is on a frame similar in size to the Colt SAA so some people prefer it's handling qualities. All New Model 357s are built on what amounts to a 44Magnum frame, and are hence stronger but bulkier.
Old Model 357s can be converted safely to 44Special, but that's about as big-bore as you should go with those. You CAN in theory convert to 45LC, there's barely enough metal to do so, but this would be highly unsafe and most gunsmiths won't. What you'd get is a Ruger in 45LC that *cannot* handle the 45LC+P loads marked "Ruger ONLY" by Cor-Bon, Buffallo Bore and the like. Plus the cylinder would be on the short side for some loads, I think? Point is, these are sweethearts in 44Spl as the amount of metal removed in the cylinder and barrel makes for a light, easy-handling gun and with the right loads and range, can still take down anything this side of a Griz or moose.
The other big difference: on an Old Model, originally they lacked transfer bars although Ruger has been updating them and will update any non-transfer-bar gun you have for free. Even with the transfer bar ignition, the loading drill on the Old Model is more "SAA-ish": you half-cock it THEN open the loading gate, at which point the chambers line up with the gate on each click. Many folks prefer that to the New Model loading drill of "pop the gate open, hammer down, but then rounds don't line up on the click". Power Custom sells a retrofit hammer/trigger set to give New Models the Old Model loading system (but still retain the New Model drill if you open the loading gate with the hammer down).
