Generally speaking a stiffer boot will be more responsive.
A softer boot will be less responsive. It is preference to a point I tend to like softer boots but many here ride stiff boots exclusively.
From the description you gave of the riding you do the driver X would work fine. The biggest thing is fit more then anything else. Dont get too hung up on stiffness, almost all boots get softer after riding in them a bit, coming from ski boots I think the saleman was trying to say that to get the response you are used to the driver X is a good buy.