I apologize for answering this question as it was directed to Stephan. I thought since I had done this myself before my answers might help.

Where to fly, Frankfurt or Dusseldorf?
Frankfurt is the bigger airport, they usually have more flights and more airlines. Dusseldorf is closer. Should you fly to Frankfurt you would need to take an ICE train ride (about 1.5 - 2 hours) north. Check with the airport (their websites) and your airline how the Icelandic ash cloud may affect your trip. Even after they cleared the airports my flight from FRA was delayed 2 hours due to the back log of flights forced to use other routes to avoid the ash. After the big shutdown it appears the airport authorities agreed on levels of ash to determine if they would shut down an airport. Sort of like fog, it depends on the density of ash.

I flew to Dusseldorf a few times but found flights to FRA cheaper. Not all airlines fly to DUS.

If you have limited time and days, by all means going to Dusseldorf will save you time.

Villa Vertigo is located about 90 meters from the Grefrath train station. I think Stephan would be better suited to address taking the train as this might mean several train changes to get to the club.

Dusseldorf clubs?
It depends how much time you have and again how you plan to get to the clubs? Options? These clubs may not all be in Dusseldorf but are popular stops for guys in the area. They are within about 40km of Dusseldorf.

Villa Vertigo
Golden Time
Living Room Close
Samya
Acapulco
Atrium Close
Dolce Vita Close
Blue Note Close

If this is your first time north (from Frankfurt) then your in for a treat. Lower entry fees, lower session fees 50 first 30 25 second 30 = 75 Euros and some girls even keep charging 25 per 30 minutes after your first 30 so it gets cheaper for longer sessions. Example 2 hours is 125 Euros
in Frankfurt its 200 Euros. Extras are also more reasonable at 25 Euros rather then 50, example BBBJCIM is only 25 more up north. 50 In Frankfurt.

Stephan's Villa Vertigo is about as good as it gets, high class facility, lots of English speakers, good staff and Stephan I'm sure can point you to the right gal. Service you won't find at the other clubs.

When most people refer to the Dusseldorf clubs it mainly since DUS is the closest big city, most of the clubs are located just outside DUS in small towns. Hope this makes sense. Like Frankfurt Oase or World are NOT in Frankfurt but Frankfurt is the nearest big city so they refer to that city. So don't expect all these clubs to be within city limits.