What strategy to adopt?

Hi!

I'm new to IM and before I fully launch myself into it, I need your feedback to help me decide what direction to take. In the past, I have spent dozens of hours on diverse projects, to end up by realizing that the project was not in line with the big picture (what I hoped to achieve) and I would just leave it there. So, learning how precious time is, I want to make sure I'm building the foundations at the right place!

My question is:
Should I focus on one site or create multiple micro-niche sites?

I write mainly on alternative health care (being myself a practicing DC) and I sell ebooks that I wrote, amazon products, and will soon integrate affiliates products.

Let say the main site is Postural Training
The micro-niches could be:
Improve posture for lower back pain
Core training exercises
Good sitting position
Ergonomic pillows for good sleeping posture
And so on..

Should I
A) create a blog/site for each niches using the keywords in the url address?
Or
B) do I create sub domains or categories in the main site (postural training)?

Using the B option, I would do my backlink work (article directories, videos,..) linking back to the category concerned (or the sub-domain)

Option A would be a small 7 articles site that I would do my backlinking to.

I guess, the question is what would be easier to rank on google for 'Good sitting position':
trainingposture.com/good-sitting-position OR good-sitting-position.com
with the same amount of backlinks.

Option A seems to offer the possibility to build a richer site with more backlinks (probably better page rank than a small 7 pages site?), but is it harder to rank than small sites that are keyword focused?

Thank you for reading me through and for your help
Moutassem