Sodium carbonate is Na2CO3 and sodium bicarbonate is NaHCO3. In other words, both are salts of the carbonate ion (CO3)2-, but in one it is bound to two sodium ions Na+ and in the other one sodium ion and one proton H+.
If you are adding the salt to increase carbonate hardness, there is no difference between the two. If you are adding it to raise pH, carbonate will have a stronger effect than bicarbonate. Remember that low pH indicates a high concentration of protons H+, and you are adding some with the bicarbonate. If you dissolve the salts in pure water, sodium carbonate you will give you a pH of 10.6, sodium carbonate will lead to a pH of 8.3.
Just googling the terms, I found a wonderful article
about the carbonate system, that I can warmly recommend if you want more details.