The biggest problem with a spring unit, or sometimes referred to as a marshall unit, is not so much the spring itself, but is the 1" or so thick foam that encases the spring unit. This 1" foam that covers the unit on top/bottom is what breaks down first, as its usually not of a high density type foam. This foam on both sides equal to 2" of seating support that once it breaks down and along with the normal softening of the polyester fiber or down/fiber blend wrap, makes the spring cushion a less durable cushion compared to a high density HR foam cushion.
If the spring unit is made from a larger stiffer wire, with either a very high density foam topper or a high density polyester pad then the spring unit will outlast a HR foam of same thickness.

rmanbike, thank you for your very helpful and explicit response to my question. I appreciate it!

One of the nice things about a spring cushion is the fast recovery of the cushion when you get up. The cushion tends to hold it's shape better than other cushions. Rmanbike is right make sure the foam that surrounds the coils in the cushion is a high quality foam. The coils within a spring cushion can also vary in their strength from soft to firm.

Another option is a spring down cushion which has several layers typically in this order, coils, fiber, foam, fiber and a down topper.