I have stayed in both the Peninsula Excelsior and tne Allson, and both of them are fine.

The Excelsior is a bit more conveniently situated.

Also if you are using Singapore Airlines (the best by far in Asia and even the world) then you could perhaps make use of the Airlines Singapore Stopover packages.

These packages include discount rates at 3 to 5 star hotels, as well as discounts or free entry to a lot of the tourist attractions. Two people can stay for S$60 a night, which is not bad for a 3 star hotel.