Walk ups are all one bedroom apartments. There must be ~50 listed in sex141, with pictures of the entrances, maps, etc. The reason for the single units is to avoid trouble with the law- it's legal to be a prostitute, but not to be a pimp.37 Dundas and KP are brothels. You go in and pick out your girl.
Also very common in Mongkok are "call-in" places, recognizable by the multi colored florescent bulbs outside, and often signage in chinese listing nationalities and prices. These places are a group of tiny bedrooms with shower, where the proprietor calls in a girl that meets your budget and description. Downside is it takes ~15 minutes for the girl to show up...
For the hours that you will be there, the brothels, walk-ups and call-in places are your best bet. Clubs will be quiet at that hour, and a longer negotiation process. Hotel reservation girls are the best looking, but need a local phone and SIM. It's very easy to get both at a 7-11, even at the airport.
As far as I have ever been able to tell, all the walkup places are the same- 6-7's, late 20s+ girls that give similar service. Others report better mileage, so it's definitely YMMV.
Dundas has better looking girls, but poorer service, and really grungy rooms and neighborhood. KP girls are similar to the walkups, but better services. The call in places really vary- I've gotten some of the best services there (rimming, anal, etc), and other times cold fish or older girls- definitely depends on your luck and willingness to wait, because you can turn down the girl, but it takes another 15 minutes to get another...

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