These days there aren't really many differences, apart from the exam structure - ACCA can be a little more flexible with regard to study. Some people regard ACCA as a broader business syllabus, but I am not convinced that is correct. ACA carries more prestige, is regarded as being the more difficult qualification to obtain and in general attracts a higher level of salary. Some of the bigger companies and accountancy firms will insist on ACA.
If you are planning to work abroad then ACCA is a more international qualification.