The answer Annie gave is very biased I think, so let me give a more unbiased answer: it depends what you are looking for.
Taiwan has more GDP per capit than China. People on average earn more money, its a more “free” country, very developed infrastructure, you are allowed to visit any website, you are allowed to speak out, you are allowed to be gay, transgender and feminist. People are very friendly and polite. Its one of the most open minded countries in Asia.
However. Mainland China has more opportunities for businesses and startups and you have the opportunity to be better off than Taiwan. Theres a reason why not all rich mainland people move to Taiwan. Because China has vast amounts of wealth for those who work hard to achieve it. If you have a successful business in China all those things I mentioned that you cant do in Taiwan? Doesnt matter anymore. Once you get to a certain level of wealth and influence in China the government acknowledges that and lets you get away with alot more (except if you want to overthrow the communist party or something). You can take advantage of the corruption in China to get what you want. You may also appreciate that China is bigger, more diverse and has places to explore, lots of beautiful scenery etc. Take note that Taipei only has 2.8 milliom people and the natural beauty there, is limited to that island. China’s biggest city Shanghai dwarfs it with 18 million people. It has vast mountains and rivers and lakes from Xinjiang to Tibet to Yunnan to Hainan to Jiangsu. Xian has thousands of years of history. Taiwan is more recently populated.
Thats my unbiased answer. (Personally as a social conservative though I prefer Korea to either China or Taiwan though)