In terms of the level of turn-out, our experience has been that it's not particularly been affected by the nature of the election as much as by the degree to which the individual elector feels that their vote is for something important. It's whether their vote counts—not in terms of its role in the proportional system, but whether their vote has a say on something that's of importance to them.
That was brought home particularly during the Scottish independence referendum when there was a turnout of over 85%. That was unprecedented in electoral events in Scotland and was reflected in the degree of engagement that people had with the topic.
Often it's the degree of engagement in the topic that's at debate rather than the process itself that affects turn-out.