microsites are hard to rank nowadays because they can't have enough content for google to consider them authority sites, and google loves authority lately. It's not impossible to rank microsites, it's just harder... typically...
So, I would recommend you build a brand / business / company website and add a blog to it.
But this advice is a general advice, your niche might justify going after micro sites instead of normal sites (main sites as you call them), so you need to do some research to see which type of site fits your niche / industry better.
For example, if you only find a few dozen to 100 keywords that are related to your niche, that have search volume, I would definitely go for a microsite regardless of how many millions of long tail keywords (long tail keywords = keywords that are 5+ words long) you still find in that niche.
On the other hand, if your niche has 1000s upon 1000s of keywords that have search volume (even if it's only 10 monthly searches, it's still good) then I would set up a normal website (actually, I would set up a blog, but that's me! If you have an e-store, a construction company, a lawyers firm, etc... you would definitely set up a normal website and attach a blog to it just for the extra traffic and rankings until you build up the authority of the main website)
Again, it comes down to researching your niche and understanding your options and the type of website will pick itself up most of the time.I
