(SEO and) Multiple blogs on 1 domain VS. Separate domains for every blog

I have some affiliate programs, as well as niches / sub-niches / microniches, and some other names related to these programs / content that I want to use to find the best keyword and try to sent some traffic and hopefully get some conversions through Wordpress blogs. I have some money that I can invest in domain names, but I was also thinking about getting just one and running separate sites under one domain (possibly Wordpress multisite).

So instead of:

domain1-with-keywords.com
domain2-with-keywords.com
domain3-with-keywords.com

I would have something like this:

domain-with-no-keywords.com/keywords-related-to-blog1
domain-with-no-keywords.com/keywords-related-to-blog1
domain-with-no-keywords.com/keywords-related-to-blog1

I am wondering how much of a difference this would make, and if it would be better to invest that money up front, or just stick to version two with only one domain, and see how it goes (in the and I can stuff with with 10 - 15 blogs etc). I would like to use keywords in the name (sub-sites, just like I described above <keywords-related-to-blog1> etc), then go down to titles, meta descriptions, h1 and h2, keywords in the content, image names, alt descriptions etc. I would like to have probably around 5 posts per page (150 - 250 words each), and then go down one level (page depth) and have some more specific keywords over there too (so the top site would be: ford, chevrolet, audi; site below it: ford focus, chevrolet cobalt, audi tt etc; I may even go one level lower to: red ford focus, red chevrolet cobalt, red audi tt etc.)

Thats basically what I want to do. What is the difference, in the long run (probably after 3 months or so) of haveing separate domains vs just one domain with multiple blogs (I want to keep the content fairly fresh etc too, maybe update it with new post once every 3 days or so)?

Thank you.

 

Go for keyword URLs. That's what I would do. Domain names are virtual real estate. The more you own the better! I'm not sure why anyone would want to complicate this process anyway. Why not just use a separate domain for each keyword?