With the subdomains, Google treats them as separate sites. So the backlinks that you build only help the specific subdomain they're pointing to.

If you create pages under one domain, then the backlinks that you build help the link profile of that page, but also contribute to the link profile of the domain.

So, between the two, you'd have a slight SEO advantage using the single domain with individual pages.

i think to make a sub domain using your keywords is a good idea.

You are MUCH better off in almost every instance, to place content under your main domain instead of under sub-domains. Sub-domains really only make sense IMO if you have enough content to build out another entire site under the sub-domain... and even then I think you're better off putting it on your main domain.

As kposs said, sub-domains are treated as separate sites. As such, they do not inherit any domain related ranking factors from the parent domain such things as domain authority, domain trust, #unique domains linking to your domain, # inbound links to your domain, etc. Each one of those sub-domains will have to start from scratch like a new domain building it's own authority, trust, etc.

Placing the content under your main domain means it will get the benefit of all of your domain's domain level ranking factors. The content will also contribute to the overall authority of your main site.

Domain authority is a factor of the number of pages on your site that have backlinks. A site with 1000 inbound links with almost all of those links pointing to the home page is going to be seen as much less of an authority than a site with 1000 inbound links spread over hundreds of pages on the site.

If you think about what an authority is... it's someone who knows a lot about all areas of a particular niche. They can answer almost any question you have about a particular topic. Now transfer that definition to the web.

A web site is typically about a particular niche. Each page on that web site "should" be about some different topic within that niche. If lots of sites are linking to lots of pages on your domain, then webmasters on other sites are telling the search engines that your site has lots of good content about lots of topics within your niche. If the same number of sites link only to your home page then it doesn't say much for your being an "authority".

I would definitely go with pages on my main domain vs. a bunch of sub-domains.

keywords are just a myth. If you over use them or increase there density to much you are on the spam side. My concern at the moment is more on the links.