From your prior posts, it seems like you want to go into Science or Engineering. Engineering pays more in general (unless you decide to go into Medicine or a related field, like Biochemistry, or Biophysics, which you cannot really do in 5 years).
Also, knowing more about your situation would help. In five years, to not have any other options other than Math, English, and Mandarin seems a bit extreme. There will be Science, Engineering, Communications, Public Speaking, etc., no? In addition, framing the problem as HAVING to chose one also seems a bit extreme. You will need all three of Math, English, and Mandarin in the real world (assuming you are staying in China). It would really help to know what sparked your original question.
To be perfectly honest, untypable, relying on China becoming enough of a power in your life-time to make English obsolete is a bit much. English has been established as one of the chief languages of Science by two empires (the British and the American).
Learning to write well is important. As you mentioned, some amount will be needed to get into university, hold particular positions at your job, etc. We all have to do things we don't like just to get through a few requirements.
Depending on where you work, and what type of job you have will determine how well you need to write in English or Mandarin.
Choosing Science, or Business over Engineering will generally require better writing skills. Engineers at lower levels don't necessarily need to be good writers. However, at higher levels in the job, writing is a must. If your company is international, English will likely still be a major language of correspondence.
When it comes to Mathematics, like nightning said, the most practical thing to study is Statistics, and often Statistics is its own separate department because of this.
Beyond all this, even if you get a high paying job, what important is that you like what you do, that you like your coworkers and employers, and that you are appreciated as an individual (trust me on this). So ultimately, you ought to do what you like. From what you've posted, English writing isn't your strong suit, and you don't seem to like it...so do only enough of it to get by, or learn to like it. You will likely need some amount of it for life.