I would say that Supernatural is an expansion pack that carries a lot of items that enhance gameplay including new plantables since a lot of potions require herbs and other items. The gem cutter is also very useful since it allows you to cut your own gemstones which helps since you don't have to spend simoleons sending in your gemstones to be cut for you; nowyou can do it yourself.
Supernatural is jam-packed with life-states; a new world (Moonlight Falls), Interactive objects. And all sorts of things that will enhance gameplay.
I did however get Pets shortly after and it added its own elements of gameplay.
Pets are able to search for things: cats search for small animals which you can sell to earn simoleons while dogs will search for gemstones or ore - my dogs can find high-value gemstones quickly but you have to train your animals to hunt first to get to that point where your dogs can enhance wealth and your cats can “decimate the wildlife population”.
You also get new forms of wildlife, Wild horses...in the Sims with Pets. You get wild horses, snakes, birds, rodents etc.
There are a few interactive new objects. A useful one is the automated pet washer which is good for getting rid of fleas on your pets when you don’t want to give them a bath. They can go remove their flea-ridden state themselves.
You also get a new world - Appaloosa Plains
I’d suggest watching LGR’s expansion pack videos on each of the Sims 3 expansion packs. He does a wonderful review of each expansion pack complete with dryly sarcastic humour.
Believe me it’s a tough choice as to which one. I ended up with both eventually. Both are highly recommended and both enhance gameplay significantly. It all depends on your interests and play-style. Do you like playing supernatural life-states and playing with magic or would you want a canine, feline or equine companion will probably be your question that you need to answer.
Both expansion packs are worth it and its hard to make a choice between the two.