If you want to do CA then b.com (//b.com) would be preferrable. As the subjects in b.com (//b.com) can be studied with reasonable efforts while doing CA . In b.com (//b.com) you have subjects like corporate accounting , taxation , mathematics etc which are almost same that you study in CA . So atleast there will be a bit less  burden on you and you can devote more time and efforts to do CA.