I would go for either Rendezvous or Marina Mandarin. These hotels are around a 15 minute walk apart, so no bad choices really, The Copthorne is an older property in Robertson Quay, a more residential area away from public transportation.

Hotel Rendezvous has just been renovated (the lobby is still being renovated I think) and is very central - less than a 5 minute walk to Dhoby Gault subway station, which is on 3 different lines, and a 5-10 minute walk to Raffles Hotel and the CHIJMES entertainment complex. Hotel Rendezvous is slightly less upscale than the Marina Mandarin, but is surrounded by some interesting colonial buildings (the Singapore Art Museum is practically next door and the Singapore Museum is 5 minutes on foot). I like this location, but then I live around this area so am biased!

Marina Mandarin is on the subway as well (Esplanade Station is closest, though City Hall is also accessible via underground tunnel). Views from here will be better, and it is closer to the Singapore Flyer. If you want shopping, then Marina Mandarin is joined onto two large malls, while Rendezvous is a short subway ride (or 5-10 minute walk) from the nearest malls on Orchard Road or at Raffles City. This is also a much larger hotel, so facilities such as pools will be more extensive.

Neither is especially well located for hawker centres, though the Rendezvous is very close to a small 24 hour food centre (part of a chain called Kopitiam) and the Esplanade area does have a few food stalls.

Neither is a bad choice, so I would check out the photos and choose the one you prefer the look of.