How to properly say "World's fair"
Wikipedia:

世界博览会（英语：Universal Exposition, World's Fair），又称国际博览会及万国博览会，简称世博会、世博、万博，是一个具国际规模的集会。参展者向世界各国展示当代的文化、科技和产业上正面影响各种生活范畴的成果。

Basically just full term vs. 简称.
世界博览会 get's shortened to 世博会, 世博
万国博览会 get's shortened to 万博
here's some quick data from baidu: (baidu hits, you know...like g-hits)

世博会: 百度为您找到相关结果约94,100,000个
世界博览会: 百度为您找到相关结果约41,300,000个
万博: 百度为您找到相关结果约68,900,000个 (but seems to have a lot of different meanings)
万国博览会: 百度为您找到相关结果约2,710,000个
国际博览会: 百度为您找到相关结果约51,800,000个

世博会 is obviously your best bet. (that's what everyone was calling it when it was in Shanghai...09(?))
Going around saying 世纪博览会 instead of 世博会 would be like someone going around saying the World Wrestling Entertainment instead of WWE or National Football League instead of NFL...