I would choose the JW Marriott over the Renaissance. The JW Marriott has far the best service, is luxurious, well maintained, right in the heart of the shopping area, the executive lounge is great and the staff is always attentive.
In Singapore we don't stay in the JW Marriott anymore because we think it's a bit overpriced. The hotel itself is great and the best thing is the location, right on Orchard Road though.
I also would suggest reading hotel reviews on Tripadvisor.com.