Note that the 0.9 family is not network compatible with the 0.8 family (Just like 0.8 vs 0.7 or 0.7 vs 0.6). So the older 0.8.8 is still listed on the website just in case you get that one player who can't get 0.9x to load.
1. Improved terrain management, eliminates lag during design portion. Also only small portions of the terrain load during transactions with the Graphics accelerator, which is easier on less powerful systems.