Prepaid → Postpaid

Ữ mobile really has a bad coverage. When you are trying to make phone call or what ever call or message with U mobile, sometimes ( cannot be said as all the time ) they will come out with a strange conversation <>. I was so angry every time because of this. Then, i decided to phone to U mobile to find out why like that. Do you know what's their reply? Erm.... how do i know? . I was so surprised that they would give such stupid answer. Can you imagine that ? my face is just as like below.

So, i decided to change my service provider to either Digi or Maxis. Can you suggest me which one should i choose ?