"Tony" wrote:
In my experience, it's a matter of how cross controlled
you have the airplane.

Exactly.
And that depends on what you're using the slip for -- to lose altitude
or for x-wind correction.

In a sideslip for altitude loss,

Side slip is used for x-wind correction, not altitude loss.

one isn't concerned about that -- you want to present
the side of the airplane to the wind, making it areodynamically
dirty.

True.

If you had some sort of monitor on the contols and reviewed their
position after the flight you'd have a hard time telling the
difference, unless you noted the controls were a lot more active if it
was for a cross wind landing.

Describe "more active"?
The controls are used to a lesser degree in a side slip for a crosswind
landing than they are in a forward slip for loss of altitude. If you
have the wing into the wind and are using full rudder and still can't
keep the axis lined up with the centerline because of the strong x-wind,
you're probably in conditions that exceed the x-wind limit of the
airplane.

The reason for pointing out the difference in the first place was to
note, for example, that if you are in an emergency where you HAVE TO
lose altitude and get the plane down quickly to a designated spot, a
"side slip" (axis lined up with the runway) isn't going to burn off
altitude like a "forward slip". Knowing the difference in the two slips
and how/when to use them in an emergency could make the difference in a
successful landing and not ending up where you wanted to.