Microniche adsense sites or One big Site with content?

i hv made some micro niche adsense sites.
That makes good amount for me.
But what if i Make a Grand Site, search for The good keyword, write the article and do SEO for that exact page for new keyword?
I mean what if i dont make new website for each and every keyword, but Search for keyword, Write article about that, then DO the same which im suppose to do for micronich sites. ( article writing and bookmarking etc)

I mean , A grand website with 20 Articles on spacific keywords, or 20 micro niche websites?
please shed som light on the idea

Not just the money but the time as well.