On crosswind landings, is it preferable to side slip or crab on final approach?
Neither technique is &quot;preferable&quot; - the assumption that one is superior to the other is one of the most common false dichotomies (and over-simplifications) in aviation.
With some notable exceptions (like the Ercoupe which is incapable of a slip and lands crabbed) a proper crosswind landing should involve elements of both techniques.

Crabbing
Crabbing involves turning the nose into the wind so that some component of the aircraft's thrust is counteracting the crosswind, allowing the aircraft's ground track to align with the runway.
Advantages
Flying on final it is advantageous to crab into the wind: It is aerodynamically more favorable than a slip, and as the aircraft is kept coordinated it is more comfortable for passengers than a slip.
Pilots often find a crab subjectively &quot;easier&quot; to maintain for extended periods of time compared to a slip: It does not require the pilot to maintain &quot;crossed controls&quot;, and it's similar to how we maintain our course in other phases of flight (you were probably crabbing into the wind on downwind just a few seconds ago, and on instrument approaches flying the localizer in a crab is analogous to tracking a VOR on the enroute portion of your flight).
Disadvantages
On touchdown however landing in a crab will put a side-load on the landing gear (the aircraft is traveling along the runway centerline, but the gear is not aligned with that direction of travel). In extreme cases this can damage the tires, cause difficulty controlling the aircraft, and possibly even damage the landing gear assembly itself.
&quot;Kicking out the crab&quot; to align the landing gear with the runway requires an abrupt control movement at low altitude (generally not a good thing), and if done too early the crosswind will begin to blow the aircraft across the runway (leading to a landing that's not on the centerline, a side-load on the gear, and in the worst case an unintended departure from the runway surface.

Slipping
Slipping involves banking the aircraft so that some portion of the wing's lift is counteracting the crosswind. Ordinarily the aircraft would tend to turn in the direction of the bank, so opposite rudder is applied to prevent the aircraft from turning and maintain the ground track parallel to the runway.
Advantages
Touching down on the runway a slip has the advantage of allowing you to align your landing gear with your direction of travel (down the runway) and avoid side-loading the gear. It also allows you to touch down on the upwind landing gear first which helps to stabilize the early roll-out, and positions your ailerons appropriately for taxiing with the crosswind.
Disadvantages
Slipping down final however is likely to make your passengers uncomfortable - the plane is slanted in one direction, the rudder is pushing the nose around in the other direction, and folks start to feel like their butt is sliding out of the seat.
Aside from passenger comfort a slip is a terribly inefficient aerodynamic arrangement (you're essentially trying to fly sideways), and not a &quot;normal&quot; way to fly in other phases of flight. You may have difficulty estimating the aircraft's performance and require power to maintain your glide path in this configuration.

Combining the techniques
So a crab is advantageous on final (and flying an instrument approach), and a slip is advantageous on touchdown. It logically follows that the two techniques can be combined to provide the advantages of both:

Crab down final (or on the localizer).
At some point transition into a slip.
Exactly where you make this transition is pilot preference, but the transition should be smooth, simultaneously bringing the nose around to align with the runway while banking into the wind to counteract the crosswind component.
Touch down in the slip.
Complete your rollout with the appropriate crosswind aileron deflection, controlling your ground track with the rudder as needed to prevent weathervaning.