Sorry for this late response...hope it gets to you in time.
From Cusco to Puno I went first class,and on the return I went in the backpacker class.
In the first class section there is a senic viewing coach where you can stand in the back which is open.
I liked the service in first class, but on the day I went it was crowded with other tourist. On the return in backpacker class there was only a handful of us. The bench seats were comfortable, the menu the same (you just had to pay for the food), the staff the same as the first class. The best part it was not crowded (yes...everyone was taking the bus back)so it was a very comfortable ride with lots of room to move around.