Thom,
Both of them are great, but I would go with the C&TS. It has a much more realistic and layed back feel than the D&S. You get to walk around the rail yards, and there is a lot more interesting equipment and structures open to the public than the D&S. Also, the scenery is much more diverse, and in my opinion more spectacular. The locomotives didn't seem to send as many cinders into my eyes as the Durango & Silverton's either. Thats my opinion, I hope it helps make the choice.
Thanks Again, Taylor