Both are different courses. IAS is known as Competitive exam whereas CA is a Course type.
After clearing the IAS exam, students go for 15 months training and that's the main part of IAS where actually they get all practical administration knowledge and thereafter their performance is get evaluated and again one rank list is published for posting.
So we cannot say that they only study. And when they get posting they should be updated with all the spheres because its a Public Administration wherein you require wide knowledge i.e. India - International Relationship, State & District knowldge (include kind of people, their behaviour, problem, how to overcome on that, etc., etc.)
Looking at the syllabus, CAs are having very huge syllabus and we claim that our syllabus is toughest....accepted. But one subject in IAS is very very lengthy i.e. GENERAL STUDIES, no one knows what they will ask. And CAs are very very weak in this area. You need daily updation of all the happening in all spheres.
In Public Sector IAS is the toughest however in Private Sector CA is the toughest!!
However, IAS is known as the toughest examination of India!!