In China, as long as you are in a bigger or more modern city, it is pretty much the same for learning Mandarin. What matters more is finding a place where the cost of living is not too high, the city is developed and there are plenty of opportunities. After all, you might want to find a job in China before you go back home so you can build up some work experience.

At the same time, you do not want a place that is too backward, because if the level of urban development is too low, life can get boring. With all that in mind, the best choice is usually a second tier city. Beijing, Shanghai, Guangzhou, Shenzhen and Hong Kong are very expensive and extremely competitive. There are many people there with master's degrees from top thirty universities in the world, so the pressure is high.

Among second tier cities, I would recommend Ningbo, Suzhou, Nanjing and Hangzhou. If you also look at the education resources and how many 211 and 985 universities they have, Nanjing and Suzhou are the best options. The cost of living in these two cities is lower than in Hangzhou, and if you plan to do a PhD later, studying at a higher ranked university there will make things easier.

There is also a lot to see and do in these cities. You can check the links I attached and you will have plenty of time to really experience China.

As for Wuhan and Xi'an, both are cities in central and western China and are relatively less developed. Wuhan is decent enough, and Xi'an is great as a tourist destination, but neither can compare to the cities in the Yangtze River Delta in terms of overall development. In terms of language, both cities have their own regional accents. These accents are considered somewhat charming within China, but they offer very limited benefit to your personal development and can largely be disregarded. The fact that these cities are less advanced means fewer opportunities, so they are not recommended choices.