We do not recommend having students choose which resources or activities they do and don't do in Math Units. The teacher should select the tasks that students do, facilitating an experience that will have kids problem solving and arriving at conceptual understandings of the big ideas of the unit.
A bit more about the rationale: Math Units involve students tackling hard problems with the goal of coming to understand big, complex mathematical ideas. For that to happen, teachers need to be ready to support students as needed just in time, get students interacting with one another about the math tasks, and bring students together to unearth the ideas (novices rarely come to these big ideas on their own; teachers are the experts and are there for a reason). When students are working on different tasks, all of those practices are difficult to pull off. There is power in having the entire class work on the same rich, valuable problem; doing so gives the class something common to come together about, allowing the class to make sense of big mathematical ideas together.