Both could be equally effective, depending how the instructor applies intensity variables to the moves. Let's look at these two examples and do some comparisons:

A long cross-country ski interval - cross-country ski is performed in the sagittal plane, uses long levers, and is considered an exercise that can elicit an aerobic response. It does answer the basic parameters for aerobic exercise: rhythmic, continuous, use of the large muscle groups. The movement pattern needs to be performed with sufficient intensity to actually achieve any muscular endurance benefits. To know if you are achieving that, you need to look at heart rate. It is possible to perform cross-country ski for ages and not be in target zone! Everything we do in water involves some level of resistance training, so strength gains are possible at all times - greater gains with more power in the moves. Cross-country offers strengthening in all muscle groups through the front and back of the legs, the hips, lower back, plus arms, shoulders, chest and back if the arms are submerged.

Ski/jack combination - everything above applies. The jack adds movement in the frontal plane, so adds strengthening of the abductors and adductors. Repetition reduction (8, 4, 2) is a well-known method of increasing challenge, so you may conclude that the combo move has more to offer. However, combos can take more focus to learn, and does this detract from being able to put more effort into the move? Probably true in the learning stages, but once the combo is familiar, a participant should be able to reach the necessary level of intensity to get benefits.