It's hard to quickly summarize the different between Chinese and Taiwanese oolong teas, as both of these types of teas are diverse. Both China and Taiwan produce both greener and darker oolongs alike, and both countries produce oolongs with varying levels of roast. Both produce oolongs from many different varietals of the tea plant--a number of varietals are grown both in Taiwan and China.

One difference is that high-mountain oolongs are very popular in Taiwan. Taiwan is able to grow tea at higher elevations in part because it's farther south, and the fact that it is surrounded by ocean moderates its climate. Greener oolongs tend to be very popular in Taiwan. Pouchong or Bao Zhong oolong is also very popular in Taiwan, even though it originated in China. Most of the Taiwanese people I know drink exclusively green oolong. Taiwan does produce a lot of darker oolong, but I think it is mostly exported. China produces and consumes lots of many different kinds of oolongs; I think it is more common to drink darker oolongs in China, although the Chinese drink mostly green tea.

Some particularly popular styles of oolong, such as Tie Guan Yin (which originated in Anxi county of Fujian, China), are even produced both in China and Taiwan.

There are some varieties of oolong, such as the se chung oolongs, and dancong, which are mainly grown in China. Some of the Wu Yi oolongs however are grown in Taiwan as well, as there are places that have conditions similar to the Wuyi mountains.