Multiple sites 1 business. Advice required.

I have a site for a plumbing business with a name as the domain ie. Billsplumbing it has a good ranking in serps. I also own 6 or so domains with no sites on them yet. They are keywords though ie. plumbingnewyork, newyorkplumbers etc.
Which is the best way to utilize them? As landing pages if so how? Build a basic site, get it up in the rankings and put a redirect? Or seperate sites to dominate 1st page of search engines. Oh it's a smaller market than plumbers and new york. My main site is already in the top 6 on page 1 of google for most of it's keywords I just want to dominate. Thanks in advance

Anyone?

Put on each of those 6 sites between 40 to 50 uniques articles, get them ranked for your targeted keywords and put a backlink on each of them to your main site.

Do the above, and once it is ranked, either sell the leads you get, or sell the whole site to a plumber in New York.