Mrs R2 and I rode the Cumbres and Toltec railroad last September. Later that week we went to Durango and rode the Durango and Silverton Train. Both are really great railroads. But if you can only ride one, ride the Cumbres and Toltec. It is the best.
Toltec Gorge on the C&S route is spectacular. But the shelf section high above the Animas River on the D&S is even more so, IMO
My vote would be the Durango Silverton. Just visited Chama this last summer. May do it again if the planets align.