Registering your domain owner makes you owner of your website, that other people can't take.

Owning a domain name is like branding yourself with all the content you have in your website than a sub-domain name from siterubix. Also i agree with masirois.

Owning your own domain is important if you're concentrating on branding yourself. If you don't own your .com (or .net, .org, etc) someone else might buy it. It's like finding the perfect name for your business - you find are the founder of "My Perfect Business" and you invest in branding and advertising, but someone scoops "myperfectbusiness.com" and now your traffic is confused about how to find you.

Owning is also an investment in your business. If you own the name, the website and the email address, they all tie back to your company, your product, your brand.

If you haven't settled yet on what your business is going to be, a siterubix account is great. You get them free, they're stable and you can have as many as you want. Once you settle on a site that you're happy with try and buy the .com version. You can transfer everything over pretty easily.

A .com is more commercial and looks better then having a .siterubix.com website but as for Google and SEO if you have great content it's not really going to matter that much. It depends on what you want to do and how you want to brand yourself.