When to Use a Sub-domain Versus Just Creating New Site?

I’m having a dilemma – I go back and forth between adding a sub-domain with multisite to my existing, well-ranked site, or creating a new site.

Pros of sub-domain:

-ride on the good ranking of the existing site
-dashboard’s are in one place
-if I don’t feel like blogging on my main site, but I blog on the site in question, google will still see fresh content every day and there is no harm done.

Cons of sub-domain:

-it’s cleaner to have a separate site both as far as the domain name and…
-it’s not convoluted with irrelevant information all on the same site together
-gotta do seo all over again and all the pain of getting below 500k Alexa ranking
-gotta add plugins all over again and start from scratch on a new site

I’m a blogger and blog mostly daily at http://AngelaDiGiovanni.com/ and I’m writing my memoirs. The sub-domain in question is for the Online Serial that I will be publishing as it is written.

So what do you say, same site with a sub-domain, or a url and site of its own?

Why?

Thank you so much!
Angela
http://AngelaDiGiovanni.com/