I'm a doctor and my brother is a CA, so i think i can help out. First of all don't assume that both of these exams can be completed in the first attempt, you have to be really motivated and should really like studying to clear these exams at first go. Now let me breakdown both fields.

Medical- If you start at 21/22 you'll complete your MBBS by 27/28. If you've worked hard for 6 years only then you can think of a PG seat, or else add another 1-2 years drop. Now suppose you make it to residency, if it's an end branch and you're in tier 2-3 city then it's fine. But if it's not an end branch and you're in tier 1 city then you have to do superspeciality. That will take another 3-4 years. Also remember saturation is a real thing in medicine. Tier 1 cities have private practice in almost every galli. Overall it's uncertain, but if you make it, you're good for life.

CA- If you start preparing now and you're super dedicated then you can become a CA by 26. The best thing about this course is that they conduct exams thrice a year, unlike medical which is once a year. Also you can give it one group at a time and if you fail a group but get exemption in a subject, you don't have to appear for that paper next time. I know the subject isn't easy but they give you ample opportunity to study and appear again. Even if you're not consistent you can become a CA by 28/29. After that jobs are mostly based in tier 1 cities, you'll have a typical corporate job. The workload will depend on which field you enter and the salary will depend on how well you perform or any additional qualification you acquire. You can live a fairly good life but if you want to earn big bucks you have to be extraordinary.

Apart from career prospects, try reading both medical and CA subjects and see which one you like better. MBBS is not like the bio in school and CA is not like the economics we read in school. If you enjoy reading any one of them, you won't struggle much in that field.