If you can only watch one, I suggest you pick Amazing Universe. If you are into stars a lot though, then I think you might like The Sky Tonight. However, it was underwhelming for me.
The Sky Tonight was all about the stars, the constellation and what you can see if you look up at the sky here from the South. From what I learned, you cannot see the Big Dipper here since it can only be found in the Northern hemisphere.
I love looking at the stars, sadly I don't have enough imagination to visualize images as they used to do eons before. Libra, Capricorn, etc just look like a bunch of dots to me.
The Amazing Universe was much more impressive. You travel to each of the planets and learn cool info about them. Like for example, on some planet, the day takes longer than a year! Simply because the rotation of the planet takes longer than its rotation around the sun.
It is also interesting to know that when we look at the stars, we are actually looking at the past.
Stars, like our sun, have a lifespan. They are born, and they die as well. Now that's scary to think about. What happens when our sun dies?
There are tons of other tidbits you can pick up from Amazing Universe but one thing that really struck me was that the realization that we are just a tiny, tiny, tiny, tiny, tiny spec in the universe. We are not anything special. There's a big, big, big universe out there.
How's that for a thought when we are all so caught up in our mundane, problematic life?