For me the choice never was between Sims 3 and 4, I'm a diehard Sims 2 player. Thing is, Sims 3 and 4 both have dealbreaker elements in them. But Sims 4 is easier on the computer and accepts all the mods that I need to turn it into an enjoyable experience. Whereas if I tried that with Sims 3, it would crash even worse than it already does. The downside of a reliance on mods is of course that I miss out on events and free stuff due to the anxiety of "what's broken today?".
So I alternate between Sims 2, Sims 4 and a handful of other games at any given time. The first two hours after switching from one to the other are horror, though :D (Sims 2: "Why can't I cancel that plum table chatter? And what do you mean by that, I have to load the community lot without sims on before I can modify it?" Sims 4: "No, you stay seated right here! And you two DON'T have that sales conversation in the swimming pool! Stopp eating in the master bedroom! And on the toilet! Come BACK here at once!")