Here is an interesting study conducted in Australia on some participants in the Australian Junior Volleyball Championships. Unfortunately I can only access the abstract…
According to the abstract, the results show that skill based conditioning games resulted in improvements in vertical jump, spike jump, speed, agility, upper body muscular power and estimated maximal aerobic power. However improvements in skills were few and far between.
This was compared to instructional training, which resulted in some improvements in spike jump and speed, but strong improvements in measures of skill.
The author’s conclusion is pretty simple. Use a combination of these methods for optimal improvements in skill level and fitness.