As I recall, there's more karma options with pets and it seems like at midnight I got hit with more bad stuff with the Sims 3. With pets there are those missions you go on with the dog, cat and by yourself (I don't remember what they're called - mysteries?). There's nothing like that in the Sims 3. Of course, they're played in different towns so if you can get the Sims 3 cheap, and at this point you should be able to, it might be worth it just for that. Mainly they added more with Pets so regular Sims might seem sort of dull, but you don't have to deal with those pesky pets. One reason to get it though, more trophies!