SAP MM (Materials Management) consists of inventory management (stock transfers, stock storage, physical inventory), purchasing (quotation, vendor selection, purchase agreements, stock transports), and invoice verification. And, MM purely deals with procurement although it has basic MRP functionality (CBP) for forecasting. This also helps to forms the P2P (Procure to Pay) scenario.
Whereas, SAP SD (Sales and Distribution) handles customer quotations, goods delivery, sales orders, invoices and so on. And, this helps to form the OTC (Order to Cash) scenario.
SAP MM and SAP SD are integrated with each other and with other modules like PP, SAP FICO, etc.
Want to learn SAP MM from scratch! then here's the right video for you:
If you are more into reading, then do check out this material and on the other hand, SAP BO (Business Objects) is used for data analysis and building complex multi-dimensional reports based on ECC data (MM, SD, PP, etc.). Coming back to your question again i.e. which one is better: SAP BO is new technology and has more demand than MM and SD. But, still, the demand for experienced MM/SD consultants has not reduced. Also, BO is a technical role, and MM/SD are functional roles that require business knowledge.