Can't say that I know much about Thonglor. I do know that it does cater to the middle class, at least in terms of condos and probably entertainment, and that there are a lot of Japanese expats living in that area... and where the Japanese go so do the higher prices. I would imagine as far as the Thais are concerned it would be a younger, upwardly mobile crowd.
I reckon if you're thinking of Thonglor in some respects you're just switching one rich foreigner ghetto for another. There's also Ratchada. These kinds of places are mostly mainstream entertainment zones. Try Bang Kapi as well. If you want to get with the girlies though and can speak some Thai, have a good attitude, are brave or have the right wingman you can have a great time in the Thai scene but it works much differently. A good way to find where to go is simply keep your eye peeled whilst in a taxi at night for places lit up with Xmas lights. There are often karaoke bars where you can eat, drink or sing if you want. The ladies are sometimes available or otherwise can choose to go with you if you get along.
Usually, in a place where the ladies go with you as part of their job you will pay everything upfront to the bar and it will probably only be for short time. This short time generalisation is not always the case though. Be aware if you go into a thai place you'll be paying for the girl's time to sit with you rather than her earning from lady drinks. One street me and my mate go to where there's a number of establishments, they can sometimes charge 240 baht for an hour of her time sitting with you. It's her choice if she leaves with you. This is more of a hostess role. In a place like this they may discourage the use of the words 'mamasan' or 'papasan' and use some other term. Another place along the road has an official mamasan and they don't always charge for the girl's time to sit with you, particularly if you have a few beers then take her off to a short or long time. Nearly all the girls along this particular road are meth heads though, and there are Thai mafia about so I wouldn't recommend it publicly. That's probably why they are willing to go long time, 'cos they can try and smoke meth at the hotel which gets them all energetic and horny but has a number of issues involved where girls doing drugs in hotels are concerned.
If you are based in BKK it's not difficult to find alternative places to go than Sukhumvit but it can take a bit more effort to find a good spot. My mate has discovered an amazing, top quality fish bowl/restaurant + rooms in Bang Na or somewhere, I forget exactly, but I'm supposed to be going along the weekend after next payday so maybe I'll make some kind of a report if it's worth it or I remember.