A first visit to Panti would certainly be a fabulous addition to a visit to Singapore - and one I'm still hoping to do at some stage. There is nowhere in China close enough to Hong Kong to offer a comparable quality of "add-on". Singapore also has the Small-clawed Otters which sound pretty easy to see. As you've been to Hong Kong already John, I can see the attraction of more ticks plus Panti.
To make the case for Hong Kong m'lud ...
Hong Kong has a greater diversity of habitat and therefore overall wider range of species.Being Hong Kong born and raised I cannot resist rising to the challenge and saying our public transport is at least as good, and our taxis are certainly cheaper!
But the big advantage of Hong Kong is that December will be significantly cooler and the birding significantly more comfortable. Singapore is as hot and sweaty as a blacksmith's armpit for 12 months out of 12!
My guiding record in Hong Kong is 121 spp in a day in November many years ago, and I recently had 91 species on my
San Tin fishponds patch in a 4-5 hr visit on December 16th. Add in Mai Po and a woodland site and I'm sure I could have broken that!
Not expecting to change your mind John, but I do hope I made the decision a little harder!
Cheers
Mike