Peninsula Excelsior and Hotel Royal @ Queens are probably the better options in terms of location. Miramar's location is not the best if you only have a short stopover.
Hotel Royal @ Queens is off Bras Basah Road - you can walk 5 mins and be at Raffles Hotel, CHIJMES (restaurants and Bars), and across the road from those is Raffles City Shopping Centre (supermarket in basement, lots of good shops, and foodcourt in basement and on top floor, plus its connected to City Link Mall - more food/shops).
Peninsula Excelsior Hotel also an excellent location on Coleman Street. 5-10 minutes walk to St Andrews Cathedral, Raffles City Shopping centre, The River, Boat Quay - bars and restaurants. Its also next to Funan Digitalife Mall - plenty of shops and food outlets. I remember staying at the Excelsior in 1983 on my first Stopover, and I regularly stay at Grand Park City Hall hotel across the road now - because the area is good.
As you say, these are cheaper 3.5 star options, but are good bases for a brief stay.
Enjoy your stopover in Singapore.
Edited: 4:36 pm, January 04, 2013