Puxi is easier and closer. You can take subway Line 8 at People's Square and get off at South Xizang Road.
No, you do not have to make reservation for each pavilions. China Pavilion need to make reservation. No need for others. There are long waiting lines to some popular ones. You can make reservations to these pavilions, but not must. Anyway it doesnot avoid making lines either if the visitors acount is so large.