I agree with Happygolucky, no matter how fit you think you might be , I guarantee you a combination of the jetlag, the heat & humidity and the extra curricular activities will take it's toll! Not to mention the dreadful traffic in Bangkok which always means getting from A to B takes longer than you think it will. Also factor in the possibility of flight delays etc that will all eat into your fun time!
For a short trip, I'd go with Bangkok, gives you more choice and variety. Half of it is the experience rather than necessarily the actual deed. I mean where else can you walk up and down a street acting a complete perv googling at every second girl (only do this in the red light districts by the way, there is about 10 million decent non sex workers and citizens in Bangkok !!)
For me for mongering, the best time to be there is at night from say 10pm to 2am or so, to just experience the street life, the streetwalkers (freelancers) and the bars etc. So try and give yourself the most nights as you can. for example book the flight home early morning rather than late at night. I made the mistake before of booking the flight home at 1am, but sure I had to leave for the airport about 9-9.30pm just as the action was starting on the streets!
My favourite area to stay is Sukhumvit 15, there is actually a great hotel with that name, attached to Hooters. It's a decent 4 star guest friendly hotel and half the price of the 5 star Sofitel next to it. You are also right beside the famous "Thermae" bar (google it) and within walking distance of Nana. There is of course cheaper options anywhere between Soi 2/4 and Soi 21 is best for a short stay mongering tourist. And it's that stretch of streets that have loads of freelancers every night, it's fun just walking up and down being propositioned.
But be wary of the Ladyboys!!!
