Concession does not apply on 502.
If time is of paramount importance, MRT is better as it is much
faster. Including walking time to Suntec, it will take 50
minutes.
If not, you can take 174 and transfer to 36, 106, 111 or 857 at
Dhoby Ghaut MRT station. Total journey time will be at least 1.5
hours.