They're completely different types of satire. Spaceballs is lampooning everything, intentionally making everything seem ridiculous. You have to know the source material to even understand any of the jokes or what's going on. Galaxy Quest presents itself as a "serious" movie, and could almost stand on its own, but familiarity with the source material gives you an extra level of appreciation, as it's a more subtle form of satire. I like them both, and own copies of each. I used to prefer Spaceballs and found Galaxy Quest just silly; at some point, they completely switched places on me. Now I find Galaxy Quest to have the more enduring replay value, and the over-the-top, joke-per-minute style of Spaceballs gets tedious. But it depends on my mood, and obviously everyone's taste is different.