Are you in Singapore for work or leisure. If for leisure, MO is definitely in the better location.
If work, is your offces within the Raffles Place area or the Suntec/City Hall area. If the former, the Westin is better located, if the latter, the MO is better.
If you are going to spend the weekend in Singapore, just note that Raffles Place (Westin) can be quite dead over the weekends without the business crowd.
MO has views of the city skyline while Westin has views of the bay area.
MO is an older but very maintained hotel. I have seen nicer MO elsewhere in Asia (Tokyo, Taipei), but this one doesn't do too badly.