I'm trying to decide between the day and overnight ferry to Newfoundland. I was leaning towards the overnight, but it seems like a pretty short ride to get much sleep, even if we can score a cabin. Has anyone done one or the other and have any thoughts?

I've done both, more times than I can keep track of (I live here).
Overnight pros: you will get some sleep, especially if you go to your cabin immediately after you board. So if you're planning to drive when you get off, this can be good. The cabins are expensive, but you're not paying for a hotel (although they're NOT cruise cabins!!) And you don't have to worry about being bored for hours, if it's a rainy, rough crossing--seasickness is less likely when lying down.
Daytime pros: If it's nice, you can wander the ship, watch the coastline as you leave NS, perhaps even see whales/porpoises. There's a bar, a cafeteria, a movie. You can strike up conversations with locals/fellow tourists for useful advice.

TIPS: do NOT ignore the deadline for arrival at the dock, and DO check in online or by phone, since mechanical/weather problems can derail the schedule.

Thank you! I booked the overnight with cabins. It hurt to pay for them, but the overnight makes so much more sense with our schedule-- and I don't want to drive without sleep. Thank you for the tips about check in!