Best shock setup for MX

[sprtrx300ex]

Which setup would be better for MX.

An Elka dual rate rear shock, or an Elka SSD rear shock?

Also, can a dual rate be changed to SSD?

[honda250xrider]

if your getting an elka dual rate rear shock usually it comes ssd

[sprtrx300ex]

The one Im looking to get is just a dual rate, no SSD.