Nyal Williams wrote:

One advantage for the slip is that it has the upwind
wing low at touchdown. I prefer to have that wing
just a bit down so that the wind is less likely to
get under it on rollout. (Same reason I want that
wing just a bit down for a crosswind launch.

One advantage for the crab is that it has the wings level. I prefer to
have the wings level so that the grass is less likely to catch the wing
and cause a ground loop. Especially important when grass and wings are
both somewhat longer. (Same reason I carefully hold the wings level
during the launch.)

Stefan