Well, in order to have full comparison, you need to wait until TS4 has several EPs. If each EP comes with a new neighborhood, then your "world" won't be so tiny because you'll be able to go everywhere. It will have more content, too.
Right now TS4 offers some gameplay plus the hope for more. By now we've seen everything that TS3 will provide (which is quite a bit). I plan to switch over to TS4, but to keep TS3 on my computer for when I feel the urge to play World Adventures or explore Into the Future some more. And if I want toddlers, I have 3 families on TS2.