One important aspect to remember in all this is that you have an opportunity to view two links back in time to what were once parts of the same railroad.
If you have time to see only one, and you are interested more in the essence of how things were and get the "feel" of the narrow gauge, the answer is the C&TS.
But if you want to see some of the most spectacular scenery in the world and experience a fine trip on the "Silverton Branch" in the bargain, then it's the D&S, but you won't be able to see much of the equipment and enjoy time tramping around in the yard. There is a museum in part of the roundhouse there, so you can still get up close to some of the equipment there.
You really can't miss either way, too bad you can't do both.
If I had only one to choose for a single day, it would be the C&TS. Because it still feels like the narrow gauge I first experienced in Durango in '66.
Mike