The PC actually contains, only the address of the next instruction. It does not contain the actual instruction. Same is the case with MAR. The difference between PC and MAR exists, based on their individual purpose.During the fetch stage of Instruction Cycle, the next instruction is actually read in the following way:

1: The PC has the address for the next instruction. This address is copied into MAR.
2: The Control Unit then polls for the actual instruction stored at the address that MAR points to. This is done by sending a signal to data bus to read the data at that address.
3: The actual data is sent to the CPU via data bus, which is then stored in the Memory Data Register (MDR) , also sometimes called Memory Buffer Register. The PC is incremented at this stage.
4: The content of MDR is then copied into CIR, the Current Instruction Register. After this, the decode stage starts.