Do different types of preferential-list PR systems create different incentives for how Members of Parliament vote? To examine this, we compare the quasi-list system of Poland, where only preference votes determine which candidates win seats, to the flexible-list system in the Czech Republic, where the 5 percent preference vote threshold required to override the party ranking of candidates gives the party greater power in influencing which candidates become MPs. We analyze roll call votes in the 2007–2011 Sejm and the 2010–2013 Czech Chamber of Deputies and, after controlling for party and MP characteristics, we find that in both countries, MPs with lower preference vote shares are more likely to vote along with their party. But, when we compare the strength of this relationship, we observe substantial differences. The magnitude of this relationship in the Czech Republic is ten times stronger than in Poland, which can be attributed to the more prominent role Czech electoral rules give to the party.