BKK vs HK vs Singapore
My tuppence worth :
Plus - easy to get to, with fairly easy transfer to Pattaya and other areas. Highly organised P4P industry. Excellent photography opportunities of the non-P4P kind (golden temples, floating markets, crocodile farms etc). The atmosphere can be quite addictive.
Minus (for me) - I find Thai women very same-ish. Very pushy in main punting areas. City suffers from heavy traffic pollution. Need to avoid scams (only moderate though).
Plus - easy to get to. Geylang P4P scene can be done in two or three hours flight stopover. Easy transfer to Bali and other locations from one of the world's most beautiful airports. High safety factor and negligible crime rate (Singapore is to BKK as BKK is to Manila), and western standard amenities at relatively low cost. Reasonable variety of WGs and varied ethnicities. Easy to browse without heavy sales pressure. British football (if you like that).
Minus - atmosphere can seem slightly sterile after a few days.
Plus - choice of modern Westernised (HKK island) environments or diverse, Chinese-style but a accessible areas on Hong Kong mainland (both with different styles of punting opportunities). Seductive ambience but can be culturally challenging (depending whether you like that or not). A way of going into (cheaper) mainland China (though I've not done this yet).
Minus - a long way to go. Need to break out of the 'tourist buying path' that hits you as soon as you step off the plane, offering fairly high priced everything, designed for tourists (WG-type bars on HK island have prices to match). Cultural misunderstandings easy.