I've actually slipped back off the fence again after a few days of uncertainty and now I'm fully enjoying Sims 3 again. Granted, the Sims aren't as expressive as they are in 4 but there's still a lot keeping my interest.
Recently my game has come to life and my lots seem very busy- I've frequently been getting 8-9 Sims at the gym with most of those working out. There are tons of features I haven't even touched upon and putting Free Will back on has made me realise that Sims aren't as robotic as they seem. Family-oriented Sims autonomously hug their family, while flirty Sims frequently use pickup lines on anyone nearby. I do think I'd have fun with Sims 4, but I think I'll stick with 3 for the time being.