Taxation

Direct taxation, assuming you are working in India in a big four or a mid-sized firm, would involve a major portion of practical application of Income tax sections and case laws learned in your CA final exam. It would involve filing replies to AO (Assessing officer) for tax demands raised on your clients, filing petitions, tax planning, tax consulting for companies planning to invest in India etc. I feel it would be really interesting to apply the sections of the act and fight for your client. You can specialize in this field and start your own practise after about five to ten years as you do need a good network of peers and clients who could become your clients down the line. (Provided you do not violate the non-compete code of your employer).
Investment banking is mostly into due diligence and advisory. You advise your clients on whether to buy a particular company or not. The deadlines can be tight and really long work hours. The pay will obviously be better once you move up the corporate ladder. It also involves a lot of practical application of what you have learnt and the additional factor being exposure to a particular industry. You will learn a lot about your clients industry and the way in which it works. Great learning experience and exposure. In case you do have plans of a MBA in the future an Investment banking work experience will definitely boost your resume.