The first decision I made for myself was in fact a mistake: I was concerned about time for some of my own topics and activities that I wanted to keep, and I thought I could do without Unit 1. Scale drawings and Proportional Relationships seemed like they would be covering similar ground, and scale drawings weren’t as critical a topic as proportions. But the two units together develop the two kinds of relationships you find in proportional relations:
Fraction skills are also a concern of mine (more on that later) and Unit 1 is an important part of that sequence as well. In Unit 1, the students multiply by unit fractions and are reminded of the equivalence of dividing by 4 and multiplying by ¼. They also find 3/5 of a number or 2.5 of a number, and are reminded of reciprocals. In both Unit 1 and 2 IM ventures into fractions gently, introducing a skill with whole numbers, then using numbers that the students can make sense of mentally or with relatively easy calculations. I have tried to do that myself in the past, but it is nice to have the details thought through for me, and so far it has worked well for my students.
I have been able to make things work this year with a little extra time and attention to scale factors, but I will be using Unit 1 next year.