As has already been stated, there are upsides for both.
At least in my area, if you want to play on your school's varsity team, you will most likely play club as well. There are a couple of high level club teams in the area and pretty much all of the varsity starters for the half a dozen + high schools in the area have been playing club since they were in junior high. If all you've played is school ball, you will not have been given the level of instruction and training that the club teams get. That is the major downfall of school teams across the country. Serious volleyball instruction is severely lacking. Many school coaches know little about the game and even less about how to teach the fundamentals of high level play. (Unless, again, you're in a strong ball area like I am, where most of the coaches are players who've come up through the local clubs, played college ball, and then returned home to coach.)
Playing for school often gets you out of class though. Unlike club ball where you have to actually give up free time in order to go to practice & tournaments.
D