If you don't know what you're doing, a 6 CIM drive could show little benefit, and sometimes even hinder your drivetrain. They're useful if you're purposefully planning on optimizing for their advantages, but otherwise I'd stick with 4 CIM drives. They're just as good, and won't give you problems if you don't plan for everything (source: used two 6 CIM transmissions on two different robots - transmissions were fine but didn't get much of an advantage from them because there wasn't much correct optimization).