Welcome to the forum robowwza!
I agree completely with Audren on buying Ambitions because it gives a lot more reality in the game (except for the ability of hunting ghosts of course). You have new open careers where you need to follow your sim too so it's no longer sending them back and waiting before they will return.
On the other hand, from what I know about Generations and what I've either read here from Naga, who has the game already and from what I saw on YouTube I recommend that one also.
It gives more live in the sims and again realism. Other than that, it includes a Ambitions like career, saying no rabithole, called Babysitter where parents are dumping their kids at your place for you to take care off. If you're good at it more kids get dumped on your lot until you almost could start your own sockerteam.
So, I can say for sure Ambitions is a must. But in a few days, or week I could also fully recommend Generations.
But, it also depends on your playstyle, and what you would like to see in the game.
My favorite skill however is making nectar (because of it's complexity and it's hanging also on the gardening and handyness skill) but that's a World Adventure skill.
You know what, get them all. It's just like Pokémon: as a team all expansions are at their best.