They mean the same thing, but sometimes word order can be used to emphasize a point. In this case however, word order does not make any obvious difference. One important difference to note is that while we normally omit the definite article in front of countries, it must be included in the case of Chinese mainland. We should say: I wish to travel to the Chinese mainland. I wish to travel to mainland China. You should avoid using China mainland, since it is poor grammar.