Singapore is very clean and safe; definitely a fantastic place to live but for a tourist, it is mostly man-made when it comes to tourist attractions and if you are a history and culture buff, you won't get as much out of Sg as you would Siem Reap. Sorry SG but that's my personal opinion.
Unless you want to do a lot of shopping then SG is for you. We have malls aplenty. Everyone speaks English, public signages are in English so communication isn't a problem.
I am planning a trip to Siem Reap myself and will definitely go there if my schedule allows it because Angkor Wat is calling my name and I have to see it in person.
I like both for different things: Siem Reap has Angkor, Singapore has great food. But if I can only choose one, I'd pick Siem Reap. The temples and the stories associated with them are amazing; it's a great experience. And Cambodian cuisine is no slouch.
But the FOOD in Singapore was a pleasure. Everywhere we ate, from street food stalls to high end, was so good. Admittedly we were coming from Manhattan which has a dearth of really good Asian food that Singapore excels at. Maybe we wouldn't have been so impressed if we were coming from Vancouver or LA or SF or any Asian city. Which is the thing with Singapore: we could probably find a place in Malaysia which serves really good laksa, or maybe some restaurant in Hainan makes an even better Hainan chicken rice. There is a chance to find these things elsewhere. Such is not the case for Ankor / Siem Reap.
Another vote for Siem Reap. If you have young kids, then they probably want to stay at Sentosa for a few days. Not sure where you are flying from but can you do a stopover in Singapore before heading to Siem Reap?
We travel to SE Asia every now and then and depending on airlines, we have to switch planes at Singapore, Tokyo or HK. We usually take advantage of that for a few days of quick sightseeing for the kids.
I go to both regularly and I am going to have to say Siem Reap. I love both but if you want a one of a kind experience then it is Siem Reap. Someone mentioned earlier doing both and I agree. I don't know where you are flying from but you could fly to S'pore and then connect to Siem Reap. Btw, as of last month there is another carrier flying into Siem Reap-- Thaismile.