How best to transition a site subsection onto a new domain

Hello,

I was in a dilemma as to whether a new estore i am developing would work best with an existing site I have (which is very established, PR 5) or go along with a fresh domain.

I have decided that it would benefit me in the long run if i develop it on a separate domain. But just to avoid starting from a completely clean plate and having to build rankings all over again (that too as the products are on a very related domain), I am considering starting the site on my existing established site first and then moving onto a fresh domain after 4 to 5 months by 301ing all pages.

For example, i would start the store initially at:

myestablishedsite.com/store-keyword1

and then gradually move onto:

keyword1keyword2.com

In your opinion would this provide better results than just starting the site fresh on a new domain?

Thanks in advance.