The worst part for Simutrans is that it's always under development. (Looking over addon side)
ModTheSims2 is the best site dedicated for addons. You can easily get packages for The Sims 2 and 3. But The Sims is a static game, we have version 2, and after some years we got 3. Simutrans is always changing, but it's always the same. It's not easy to know if that addon will work.
Also ModTheSims2 have a huge moderator team, only for the addons.