I am in the process of setting up a new website, hosting related lol, and am trying to rough draft how I am going to set this one up.

The question: Is it better to use directorys or sub-domains and why?

Thanks guys!

Depending of what you are planning to do. Anyway when setting up a sub-domain you normally create a folder (it works that way with cPanel), so in that case your visitors can access using the sub domain as well as the folder. (you can also set up a redirection of the sub-domain)

Sub domains are good for certain things, for example, if you want your customers to access their account area directly you can set up one called myaccount.domain.com etc.

One consideration is that search engines will generally treat each approach differently. Essentially the idea is that each top-level page within a subdomain will appear as a separate home page, and each subdomain division treated like a separate web site -- to an extent. For example, it's not the case with how PageRank is distributed.

Anyway this presents both opportunities and dangers. A big potential benefit is that if you develop each subdomain as a separate site, you can clearly establish a them for each "website," which can be very helpful especially if the combined website would bring together two or more disparate topics.

But if you're going to take that approach you really do have to main that distinct architecture. Be careful, too, of duplicating content across your subdomains.

I guess the short answer to your question "Is it better to use directories or sub-domains and why?" is that it depends on what you're trying to achieve, and it depends to a large extent on the original site's contents. Most importantly, don't lose sight of the goal of keeping your entire property well-organized and easily navigable for human visitors. Mixing stuff up between subdomains presents a good opportunity to turn the site into a maze.

I recommend subdomains. It look's like "profi" prezentation.

Most webhosting providers supports creating subdomains by creating directory in the root of your website. Ask them about this feature.

What if what I do is I have something like b.mysite.com and in that subdomain, a page will redirect it to www.mysite.com/b/index.php? Will it be good or bad?

directory is better and fast but subdomain is professional. but remember subdomains they act like domains so they take time to resolve plus if you want to check the stats on a subdomain its different it will not go under the primary domain. most host providers use cpanel.

What if what I do is I have something like b.mysite.com and in that subdomain, a page will redirect it to www.mysite.com/b/index.php? Will it be good or bad?

That is bad..
in case if both accounts at the same server it is better to change ROOT directory at config file to be pointed to www.mysite.com/b/.