I had this choice to make at the end of March and I chose HK. I was really geared up for a move to Singapore as I'd visited a couple of times and a position came up with my company. However, the HR people in Singapore spent so long putting an offer together that I applied for a job in HK and here I am.
I should say though, the HK job paid a lot more than the Singapore one (the company offered me a transfer to Sin but after I told them I was quitting to go to HK they made me an offer but wanted me to take a significant pay-cut and work on a local salary, yeah that's really going to convince me to stay!!) so that was the big deciding factor. I'd never been to HK before and had been to Singapore so all things being equal I probably would have chosen Singapore simply because it was familiar and I do like it there.
Looking back now I've been in HK a while, and disregarding the money factor, I think I made the right choice. The weather here will definately suit me more in the winter than it would in singapore as I'm from the land without sun.
The one good thing about Singapore or HK is that they're both very accessible to SE Asia which was a big reason I wanted to work in either. Plenty of cheap airlines from singapore or HK (or macau) can get you all over Asia and if you're a traveller like me that's really good.
Oh, and go to cuisine cuisine restaurant in IFC if you want good dim sum