For a stop over, i would stay on the mainland, closer to all the activity, unless you are intent on being at a beach, in which case Sentosa is the only option.
Orchard Road area has lots of hotels, but unless you are intending to spend lots of time in high end shops, i dont think it is the best area to stay in.
I would recommend the Marina Bay area, which is pretty quick and easy from airport by taxi, and very easy to get around to the interesting areas; china town, little india, CBD. The conrad centennial is a very nice hotel, and the breakfasts there at Oscars are excellent and will set you up all day. Also handy about the conrad, is that it is opposite the main stop for both routes on the Hippo hop on hop off tour bus , which is an excellent way to quickly get to grips with the orientation and main areas of singapore.
Alternatively, the Marina Mandarin is also very good, not far from the Conrad but slightly closer to walk into colonnial district main. Both hotels are close to Marina square shopping mall which has a number of handy eateries.