there less than 2 kms apart, go to both
Cowboy soi 21
nana soi 4
IMHO Cowboy is more relaxed and a more fun area than Nana which is degenerating every year and becoming more and more just a pick up spor for sex tourists. Soi Cowboy there are of course gogo bars if you want them but at either end of the pedestrianised street there are bars where you can just sit outside and people watch if you like.
Agree with above and usually bring first timers to Soi Cowboy if they want to have a peek.
As mentioned above you could check out both in the same evening. But Soi Cowboy is a lot better than Nana. If checking out both and wanting to go into the ' dance bars ' I would start with Nana Plaza. I like Spankeys, up the stairs on the left as you walk into the plaza. Good fun atmosphere and they don't hassle the punters. The dance club next door has some good rock music. Maybe start off there about 9 or 10 pm and after an hour head to Soi Cowboy. There are two live music bars at the start of Soi Cowboy and you could sit outside and watch the tourists stroll by. I see enough tourists already and prefer going indoors. Susie Wongs has the best show in my opinion. They have a few shows in fact. The guy that does the body painting is incredible but that's not my favourite show there. But the best gogo bar I have been to anywhere in Thailand is Crazy House. Walk down to the end of Soi Cowboy and turn right into Soi 23 and it's first door on the right about 10 metres along.
One is a street (Soi Cowboy) and one is an open-air plaza (Nana). Both have the same types of bars and precautions. You can overspend in either or get some good deals. Any shows generally start at 10pm if the bar has them.
There are more ladyboy bars in Nana than Cowboy, but there are also more shows in Nana from the gogos as well. IMO I have generally found Cowboy to be more expensive especially around the Arab's bars.
As mentioned by Jagamania Susie Wongs is in Soi Cowboy